Advanced oxidative coupling of methane

ABSTRACT

The present disclosure provides a method for generating higher hydrocarbon(s) from a stream comprising compounds with two or more carbon atoms (C 2+ ), comprising introducing methane and an oxidant (e.g., O 2 ) into an oxidative coupling of methane (OCM) reactor that has been retrofitted into a system comprising an ethylene-to-liquids (ETL) reactor. The OCM reactor reacts the methane with the oxidant to generate a first product stream comprising the C 2+  compounds. The first product stream can then be directed to a pressure swing adsorption (PSA) unit that recovers at least a portion of the C 2+  compounds from the first product stream to yield a second product stream comprising the at least the portion of the C 2+  compounds. The second product stream can then be directed to the ETL reactor. The higher hydrocarbon(s) can then be generated from the at least the portion of the C 2+  compounds in the ETL reactor.

CROSS-REFERENCE

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/141,789, filed Apr. 1, 2015, which is entirely incorporatedherein by reference.

BACKGROUND

The modern refining and petrochemical industry makes extensive use offractionation technology to produce and separate various desirablecompounds from crude oil. The conventional fractionation technology isenergy intensive and costly to install and operate. Cryogenicdistillation has been in use for over a hundred years to separate andrecover hydrocarbon products in various refining and petrochemicalindustries. However, there is a need for non-cryogenic separationmethods and systems, particularly for oxidative coupling of methane(OCM) processes.

SUMMARY

Aspects of the present disclosure provide processes for recoveringolefins from a stream containing mix of hydrocarbons by utilizingtechniques based the use of adsorbents. In some embodiments, systems andmethods enable the separation, pre-separation, purification and/orrecovery of hydrocarbons, including, but not limited to, olefins,ethylene, propylene, methane, and ethane, and CO₂, from a multicomponenthydrocarbon stream such as an effluent stream from an oxidative couplingof methane (OCM) reactor or an ethylene-to-liquids (ETL) reactor. Thehydrocarbon stream can also be the feed to the OCM or ETL reactor incertain cases. In certain cases, the feed to the ETL reactor is theeffluent from OCM reactor. In some cases, a separation process utilizingadsorbents can be used to purify and pre-treat existing hydrocarbonstreams (such as refinery off-gases, cracker off-gas, streams from NGLplants, and others), followed by use of the resulting olefin rich stream(e.g., PSA tail gas) as the ETL feed.

The present disclosure provides various improvements in OCM and ETLprocesses, such as, without limitation, a separation and pre-separationprocess to recover desired or predetermined components from an OCMreactor effluent, CO₂ recovery and capture techniques, enhanced heatrecovery methods to utilize the OCM reaction heat more efficiently, andtechniques and technologies to further reduce the carbon footprint ofthe OCM process.

An aspect of the present disclosure provides a method for generatinghigher hydrocarbon(s) from a stream comprising compounds with two ormore carbon atoms (C₂₊), comprising introducing methane and an oxidant(e.g., O₂) into an oxidative coupling of methane (OCM) reactor that hasbeen retrofitted into a system comprising an ethylene-to-liquids (ETL)reactor. The OCM reactor reacts the methane with the oxidant to generatea first product stream comprising the C₂₊ compounds. The first productstream can then be directed to a pressure swing adsorption (PSA) unitthat recovers at least a portion of the C₂₊ compounds from the firstproduct stream to yield a second product stream comprising the at leastthe portion of the C₂₊ compounds. The second product stream can then bedirected to the ETL reactor. The higher hydrocarbon(s) can then begenerated from the at least the portion of the C₂₊ compounds in the ETLreactor.

In some cases, the first product stream is directed to otherintermediate units before the PSA, such as a post-bed cracking (PBC)unit that generates alkenes from alkanes. The alkenes can be included inthe first product stream, which can then be directed to the PSA.

In an aspect, the present disclosure provides a method for generatinghigher hydrocarbon(s) from a stream comprising compounds with two ormore carbon atoms (C₂₊), comprising: (a) introducing methane and anoxidant into an oxidative coupling of methane (OCM) reactor that hasbeen retrofitted into a system comprising an ethylene-to-liquids (ETL)reactor, where the OCM reactor reacts the methane with the oxidant togenerate a first product stream comprising the C₂₊ compounds; (b)directing the first product stream to a pressure swing adsorption (PSA)unit that recovers at least a portion of the C₂₊ compounds from thefirst product stream to yield a second product stream comprising the atleast the portion of the C₂₊ compounds; (c) directing the second productstream to the ETL reactor; and (d) generating the higher hydrocarbon(s)from the at least the portion of the C₂₊ compounds in the ETL reactor.

In some embodiments, the method further comprises: (e) recovering alight stream comprising (i) hydrogen and (ii) carbon monoxide (CO)and/or carbon dioxide (CO₂) from the PSA unit and recycling the lightstream to the OCM reactor; (f) directing at least a portion of the lightstream into a methanation unit that reacts the hydrogen and the COand/or CO₂ to produce a methanation product stream comprising methane;and (g) directing the methanation product stream into the OCM reactor.

In some embodiments, the method further comprises recovering C₂ and/orC₃ compounds from the second product stream and directing the C₂ and/orC₃ compounds to the OCM reactor. In some embodiments, the OCM reactorfurther comprises a post-bed cracking (PBC) unit.

In another aspect, the present disclosure provides a method forgenerating compounds with two or more carbon atoms (C₂₊ compounds),comprising: (a) directing oxygen (O₂) and methane (CH₄) into anoxidative coupling of methane (OCM) reactor that reacts the O₂ and CH₄in an OCM process to yield a product stream comprising (i) C₂₊ compoundsincluding ethylene (C₂H₄) and (ii) carbon monoxide (CO) and/or carbondioxide (CO₂); and (b) directing the product stream from the OCM reactorinto a separations system that employs a refrigeration unit having arefrigerant that includes methane from the product stream, to enrich theC₂₊ compounds in the product stream.

In some embodiments, the product stream is directed into the separationssystem through one or more additional units.

In some embodiments, the method further comprises separating methanefrom the product stream for use in the refrigeration unit. In someembodiments, the method further comprises directing CO and/or CO₂ fromthe product stream to a methanation reactor that reacts the CO and/orCO₂ to yield a methanation product stream comprising methane. In someembodiments, the method further comprises directing at least a portionof the methane in the methanation product stream to the OCM reactor. Insome embodiments, the method further comprises separating the productstream into (i) an ethylene product stream comprising ethylene and (ii)a C₃₊ product stream comprising compounds with three or more carbonatoms (C₃₊ compounds). In some embodiments, the method further comprisesdirecting ethane from the product stream to the OCM reactor. In someembodiments, the method further comprises prior to directing the productstream into the separations system, compressing the product stream.

In another aspect, the present disclosure provides a method forgenerating compounds with two or more carbon atoms (C₂₊ compounds),comprising: (a) directing oxygen (O₂) and methane (CH₄) into anoxidative coupling of methane (OCM) reactor that reacts the O₂ and CH₄in an OCM process to yield a product stream comprising (i) C₂₊ compoundsincluding ethylene (C₂H₄) and (ii) carbon monoxide (CO) and/or carbondioxide (CO₂); and (b) directing the product stream from the OCM reactorinto a separations system that employs a complexation unit having acomplexation catalyst that forms pi complexes with the ethylene in theproduct stream, to enrich the C₂₊ compounds in the product stream.

In some embodiments, the product stream is directed into the separationssystem through one or more additional units. In some embodiments, themethod further comprises using the complexation unit to remove one ormore impurities from the product stream, where the impurities areselected from the group consisting of CO₂, sulfur compounds, acetylenes,and hydrogen. In some embodiments, the complexation catalyst includesone or more metals selected from the group consisting of silver andcopper. In some embodiments, the method further comprises directing COand/or CO₂ from the product stream to a methanation reactor that reactsthe CO and/or CO₂ to yield a methanation product stream comprisingmethane. In some embodiments, the method further comprises directing themethane in the methanation product stream to the OCM reactor. In someembodiments, the method further comprises separating the product streaminto (i) an ethylene product stream comprising ethylene and (ii) a C₃₊product stream comprising compounds with three or more carbon atoms (C₃₊compounds). In some embodiments, the method further comprises directingethane from the product stream to the OCM reactor. In some embodiments,the method further comprises prior to directing the product stream intothe separations system, compressing the product stream.

In another aspect, the present disclosure provides a method forgenerating compounds with two or more carbon atoms (C₂₊ compounds),comprising: (a) directing oxygen (O₂) and methane (CH₄) into anoxidative coupling of methane (OCM) reactor that reacts the O₂ and CH₄in an OCM process to yield a product stream comprising (i) C₂₊ compoundsincluding ethylene (C₂H₄) and (ii) carbon dioxide (CO₂); and (b)directing the product stream from the OCM reactor into a separationssystem that employs a CO₂ separation unit to separate the CO₂ from theproduct stream, to enrich the C₂₊ compounds in the product stream, whichCO₂ separation unit employs (i) sorbent or solvent separation of CO₂,(ii) membrane separation of CO₂, or (iii) cryogenic or low temperatureseparation of CO₂ having an operating temperature greater than a boilingpoint of methane and less than a boiling point of CO₂.

In some embodiments, the product stream is directed into the separationssystem through one or more additional units. In some embodiments, thesorbent or solvent separation of CO₂ employs an amine based absoprtionsystem. In some embodiments, the sorbent or solvent separation of CO₂employs a Benfield process. In some embodiments, the sorbent or solventseparation of CO₂ employs diethanolamine. In some embodiments, thesorbent or solvent separation of CO₂ employs glycol dimethylether. Insome embodiments, the sorbent or solvent separation of CO₂ employspropylene carbonate. In some embodiments, the sorbent or solventseparation of CO₂ employs Sulfinol.

In some embodiments, the sorbent or solvent separation of CO₂ employs azeolite. In some embodiments, the sorbent or solvent separation of CO₂employs active carbon. In some embodiments, the CO₂ separation systemcomprises a membrane CO₂ separation system. In some embodiments, themembrane separation of CO₂ employs a polymeric membrane. In someembodiments, the membrane separation of CO₂ employs a metallic membrane.In some embodiments, the membrane separation of CO₂ employs a ceramicmembrane. In some embodiments, the membrane separation of CO₂ employs ahybrid membrane comprising a membrane supporting a solvent or sorbent.In some embodiments, the membrane separation of CO₂ employs a poly ionicliquid membrane. In some embodiments, the membrane separation of CO₂employs a supported ionic liquid membrane. In some embodiments, themembrane separation of CO₂ employs a polyetherimide membrane.

In some embodiments, the method further comprises directing the CO₂ fromthe product stream to a methanation reactor that reacts the CO₂ to yielda methanation product stream comprising methane. In some embodiments,the method further comprises directing the methane in the methanationproduct stream to the OCM reactor. In some embodiments, the methodfurther comprises separating the product stream into (i) an ethyleneproduct stream comprising ethylene and (ii) a C₃₊ product streamcomprising compounds with three or more carbon atoms (C₃₊ compounds). Insome embodiments, the method further comprises directing ethane from theproduct stream to the OCM reactor. In some embodiments, the methodfurther comprises prior to directing the product stream into theseparations unit, compressing the product stream.

In another aspect, the present disclosure provides a method forgenerating compounds with two or more carbon atoms (C₂₊ compounds),comprising: (a) directing water into an electrolysis unit thatelectrolyzes the water to yield oxygen (O₂) and hydrogen (H₂); (b)directing the O₂ from the electrolysis unit and methane (CH₄) into anoxidative coupling of methane (OCM) reactor that reacts the O₂ and CH₄in an OCM process to yield a product stream comprising (i) C₂₊compounds, including ethylene (C₂H₄) and (ii) carbon monoxide (CO)and/or carbon dioxide (CO₂); (c) directing at least a portion of the COand/or CO₂ from the product stream and the H₂ from the electrolysis unitinto a methanation reactor that reacts the H₂ and the CO and/or CO₂ toyield CH₄; and (d) directing at least a portion of the CH₄ from themethanation reactor to the OCM reactor.

In some embodiments, the electrolysis unit comprises an alkaline waterelectrolysis system. In some embodiments, the electrolysis unitcomprises a proton exchange membrane electrolysis system. In someembodiments, the electrolysis unit comprises a steam electrolysissystem.

In another aspect, the present disclosure provides a method forgenerating compounds with two or more carbon atoms (C₂₊ compounds),comprising: (a) directing oxygen (O₂) and methane (CH₄) into anoxidative coupling of methane (OCM) reactor that reacts the O₂ and CH₄in an OCM process to yield a product stream comprising (i) C₂₊ compoundsincluding ethylene (C₂H₄) and (ii) carbon dioxide (CO₂); (b) directingthe product stream from the OCM reactor into a separations system thatemploys a CO₂ separation unit that separates the CO₂ from the productstream to enrich the C₂₊ compounds in the product stream; and (c)directing at least a portion of the CO₂ separated in (b) to the OCMreactor.

In some embodiments, the product stream is directed into the separationssystem through one or more additional units.

In another aspect, the present disclosure provides a method forgenerating compounds with two or more carbon atoms (C₂₊ compounds),comprising: (a) directing oxygen (O₂) and methane (CH₄) into anoxidative coupling of methane (OCM) reactor that reacts the O₂ and CH₄in an OCM process to yield a product stream comprising C₂₊ compoundsincluding ethylene (C₂H₄) and heat; (b) using an evaporator to transferat least a portion of the heat from the product stream to an organicworking fluid in a closed fluid flow cycle as part of an organic Rankinecycle (ORC) system, to evaporate the organic working fluid, which closedfluid flow cycle includes the evaporator, a turbine, a condenser, and apump; (c) directing the organic working fluid evaporated in (b) to theturbine to generate power; (d) directing the organic working fluid fromthe turbine to the condenser that condenses the organic working fluid;and (e) directing the organic working fluid condensed in (d) to thepump.

In some embodiments, the organic working fluid is selected from thegroup consisting of hydrocarbons, silicon oils, and perfluorocarbons. Insome embodiments, a boiling point of the organic working fluid is lessthan a boiling point of water.

In another aspect, the present disclosure provides a method forgenerating compounds with two or more carbon atoms (C₂₊ compounds),comprising: (a) directing oxygen (O₂) and methane (CH₄) into anoxidative coupling of methane (OCM) reactor that reacts the O₂ and CH₄in an OCM process to yield a product stream comprising (i) C₂₊ compoundsincluding ethylene (C₂H₄) and heat; (b) transferring at least a portionof the heat from the product stream to a thermoelectric power generator;and (c) with the aid of the heat, using the thermoelectric powergenerator to generate power.

In some embodiments, the thermoelectric generator comprises a thin filmthermoelectric module. In some embodiments, the thermoelectric generatorcomprises a micro thermoelectric module.

Additional aspects and advantages of the present disclosure will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein only illustrative embodiments of thepresent disclosure are shown and described. As will be realized, thepresent disclosure is capable of other and different embodiments, andits several details are capable of modifications in various obviousrespects, all without departing from the disclosure. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE FIGURES

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings or figures (also “FIG.” and “FIGs.” herein), ofwhich:

FIG. 1 shows a typical oxidative coupling of methane (OCM) system withadvanced separation;

FIG. 2 shows an OCM system with auto refrigeration (e.g., methanerefrigeration);

FIG. 3 shows an exemplary OCM system with a silver complexation ethylenerecovery subsystem;

FIG. 4 shows an exemplary pressure swing adsoprtion (PSA) system;

FIG. 5A shows a schematic of CO₂ separation methods;

FIG. 5B shows a schematic of CO₂ separation methods;

FIG. 5C shows a schematic of CO₂ separation methods;

FIG. 6 shows typical CO₂ distillation system;

FIG. 7 shows a water electrolysis sub system;

FIG. 8 shows an OCM system with CO₂ as a quench medium;

FIG. 9 shows an organic Rankine cycle (ORC) subsystem;

FIG. 10 shows an exemplary typical OCM system;

FIG. 11 shows an exemplary OCM system with a single stage PSA unit;

FIG. 12 shows an exemplary OCM system with a multi stage PSA unit;

FIG. 13 shows an exemplary retrofit of OCM to a cracker, with a singlestage PSA unit;

FIG. 14 shows an exemplary retrofit of OCM to a cracker, with a multistage PSA unit;

FIG. 15 shows exemplary configurations of ethylene to liquids (ETL)systems without PSA;

FIG. 16 shows exemplary configurations of ETL systems with PSA;

FIG. 17 shows an exemplary PSA unit integrated with an OCM-ETL systemfor a midstream application;

FIG. 18 shows an exemplary PSA unit integrated with an OCM-ETL system ina natural gas liquids (NGL) application;

FIG. 19 shows an exemplary PSA unit integrated with an OCM-ETL systemfor a refining application; and

FIG. 20 shows an exemplary alternate scheme for a PSA unit integratedwith an OCM-ETL system for a refining application.

DETAILED DESCRIPTION

While various embodiments of the invention have been shown and describedherein, it will be obvious to those skilled in the art that suchembodiments are provided by way of example only. Numerous variations,changes, and substitutions may occur to those skilled in the art withoutdeparting from the invention. It should be understood that variousalternatives to the embodiments of the invention described herein may beemployed.

The term “higher hydrocarbon,” as used herein, generally refers to ahigher molecular weight and/or higher chain hydrocarbon. A higherhydrocarbon can have a higher molecular weight and/or carbon contentthat is higher or larger relative to starting material in a givenprocess (e.g., OCM or ETL). A higher hydrocarbon can be a highermolecular weight and/or chain hydrocarbon product that is generated inan OCM or ETL process. For example, ethylene is a higher hydrocarbonproduct relative to methane in an OCM process. As another example, a C₃₊hydrocarbon is a higher hydrocarbon relative to ethylene in an ETLprocess. As another example, a C₅₊ hydrocarbon is a higher hydrocarbonrelative to ethylene in an ETL process. In some cases, a higherhydrocarbon is a higher molecular weight hydrocarbon.

The term “OCM process,” as used herein, generally refers to a processthat employs or substantially employs an oxidative coupling of methane(OCM) reaction. An OCM reaction can include the oxidation of methane toa higher hydrocarbon and water, and involves an exothermic reaction. Inan OCM reaction, methane can be partially oxidized and coupled to formone or more C₂₊ compounds, such as ethylene. In an example, an OCMreaction is 2CH₄+O₂→C₂H₄+2H₂O. An OCM reaction can yield C₂₊ compounds.An OCM reaction can be facilitated by a catalyst, such as aheterogeneous catalyst. Additional by-products of OCM reactions caninclude CO, CO₂, H₂, as well as hydrocarbons, such as, for example,ethane, propane, propene, butane, butene, and the like.

The term “non-OCM process,” as used herein, generally refers to aprocess that does not employ or substantially employ an oxidativecoupling of methane reaction. Examples of processes that may be non-OCMprocesses include non-OCM hydrocarbon processes, such as, for example,non-OCM processes employed in hydrocarbon processing in oil refineries,a natural gas liquids separations processes, steam cracking of ethane,steam cracking or naphtha, Fischer-Tropsch processes, and the like.

The terms “C₂₊” and “C₂₊ compound,” as used herein, generally refer to acompound comprising two or more carbon atoms. For example, C₂₊ compoundsinclude, without limitation, alkanes, alkenes, alkynes and aromaticscontaining two or more carbon atoms. C₂₊ compounds can includealdehydes, ketones, esters and carboxylic acids. Examples of C₂₊compounds include ethane, ethene, acetylene, propane, propene, butane,and butene.

The term “non-C₂₊ impurities,” as used herein, generally refers tomaterial that does not include C₂₊ compounds. Examples of non-C₂₊impurities, which may be found in certain OCM reaction product streams,include nitrogen (N₂), oxygen (O₂), water (H₂O), argon (Ar), hydrogen(H₂) carbon monoxide (CO), carbon dioxide (CO₂) and methane (CH₄).

The term “small scale,” as used herein, generally refers to a systemthat generates less than or equal to about 250 kilotons per annum (KTA)of a given product, such as an olefin (e.g., ethylene).

The term “world scale,” as used herein, generally refers to a systemthat generates greater than about 250 KTA of a given product, such as anolefin (e.g., ethylene). In some examples, a world scale olefin systemgenerates at least about 1000, 1100, 1200, 1300, 1400, 1500, or 1600 KTAof an olefin.

The term “item of value,” as used herein, generally refers to money,credit, a good or commodity (e.g., hydrocarbon). An item of value can betraded for another item of value.

The term “carbon efficiency,” as used herein, generally refers to theratio of the number of moles of carbon present in all process inputstreams (in some cases including all hydrocarbon feedstocks, such as,e.g., natural gas and ethane and fuel streams) to the number of moles ofcarbon present in all commercially (or industrially) usable ormarketable products of the process. Such products can includehydrocarbons that can be employed for various downstream uses, such aspetrochemical or for use as commodity chemicals. Such products canexclude CO and CO₂. The products of the process can be marketableproducts, such as C₂₊ hydrocarbon products containing at least about 99%C₂₊ hydrocarbons and all sales gas or pipeline gas products containingat least about 90% methane. Process input streams can include inputstreams providing power for the operation of the process. In some cases,power for the operation of the process can be provided by heat liberatedby an OCM reaction. In some cases, the systems or methods of the presentdisclosure have a carbon efficiency of at least about 50%, at leastabout 55%, at least about 60%, at least about 65%, at least about 70%,at least about 75%, at least about 80%, at least about 85%, or at leastabout 90%. In some cases, the systems or methods of the presentdisclosure have a carbon efficiency of between about 50% and about 85%,between about 55% and about 80%, between about 60% and about 80%,between about 65% and about 85%, between about 65% and about 80%, orbetween about 70% and about 80%.

The term “C₂₊ selectivity,” as used herein, generally refers to thepercentage of the moles of methane that are converted into C₂₊compounds.

The term “specific oxygen consumption,” as used herein, generally refersto the mass (or weight) of oxygen consumed by a process divided by themass of C₂₊ compounds produced by the process.

The term “specific CO₂ emission,” as used herein, generally refers tothe mass of CO₂ emitted from the process divided by the mass of C₂₊compounds produced by the process.

Separations

Various non-cryogenic separation techniques have been increasinglyemployed for gas separations, purifications and recovery ofhydrocarbons. Membrane based processes and adsorbents have beenintensively studied for large scale applications for olefins recovery.Since the development of synthetic adsorbents and pressure swingadsorption (PSA) cycles, adsorption has been playing an increasinglyimportant role in gas separation and purification.

PSA technology can be used in a large variety of applications: Hydrogenpurification, air separation, CO₂ removal, noble gases purification,methane upgrading, n-iso paraffin separation and so forth. While newapplications for gas separations by adsorption are continually beingdeveloped, the most important applications have been air separation (forproduction of O₂ and N₂) and hydrogen separation (from fuel gas).Approximately 20% of O₂ and N₂ are currently produced by PSA. Theincreasing industrial applications for adsorption have stimulated agrowing interest in research and new applications.

Processes of the present disclosure can employ a variety of differentseparations techniques, alone or in combination. For example, OCMprocesses can employ amine and caustic systems for CO₂ removal,molecular sieve guard beds for water removal, and cryogenic distillationor other separation techniques for recovery and purification ofhydrocarbon components. Cryogenic separation can refer to separationsusing temperature levels below 120 K or about −153° C. Other techniquesinclude Selexol™ and Rectisol™ processes for CO₂ removal.

OCM product effluent can comprise a mixture of hydrocarbons includingbut not limited to methane, ethane, ethylene, propane, propylene,butanes, butenes, and higher hydrocarbons. OCM product effluent can alsocomprise varying amounts of other components such as H₂, N₂, CO, CO₂ andH₂O. The product of an OCM reaction can include ethylene. The ethyleneproduct can be polymer grade, refinery grade or chemical grade.Depending on the purity level required, different separation and/orpurification techniques can be employed with the OCM process. To recoverhigh purity ethylene, separation methods such as those discussed hereincan be used to remove a wide range of components.

Advantages of the advanced OCM processes described herein can includereducing the cost, reducing the number of unit operations (“units”)used, and hence improving the overall process for producing high puritypolymer grade ethylene. Overall conversion and carbon efficiency canalso be improved. The separation methods disclosed herein can alsoimprove the overall conversion and carbon efficiency.

The different separation and purification techniques discussed hereincan be used to separate the OCM product effluent (e.g., process gas)into a plurality of streams, including but not limited to a first streamcomprising methane, hydrogen, carbon monoxide and other lighter inertsand a second stream comprising ethane, ethylene, propylene, and higherhydrocarbons. Separation systems or subsystems employed can includethose discussed herein, such as a cryogenic demethanizer, a membraneseparation system, or a PSA based system.

The separation techniques discussed herein can be employed to removeCO₂, such as from an OCM product effluent stream. One or moreseparations techniques can be used to remove CO₂ including but notlimited to absorption, adsorption, CO₂ distillation, and membraneseparation. The separation technique can be non-cryogenic.

FIG. 1 shows a block flow diagram for an exemplary OCM process. Oxygen110 and methane 121 can be fed into an OCM reactor 101 for conversioninto higher hydrocarbon compounds including ethylene. The OCM productstream 111 can be directed to a compressor 102, and the compressedproduct stream 112 can be fed into a separations system 103. Theseparations system can include pretreatment units 104, such as impurityand CO₂ removal units, as well as separations units 105, such ascryogenic, non-cryogenic, complexation, membrane, and other separationsunits. The separations system can be a combination of more than oneseparation techniques, such as those discussed in this application. Theseparation system can replace CO₂ removal, moisture removal, andcryogenic separation systems of existing OCM process systems. Thecompressor system may not be required for some types of separationprocesses. From the separations system, CO₂ can be vented 113, ethane114 can be recovered, for example for recycling to the OCM reactor,ethylene product 115 can be recovered, and C₃₊ products 116 can berecovered. Additionally, CO₂ 117 and methane 118 can be directed fromthe separations system into a methanation unit 106. The methanation unitcan produce methane from the CO₂, for recycling 119 back to the OCMreactor. Additional methane 120 can be added to the OCM reactor supplystream 121.

Auto Refrigeration

OCM process systems can use refrigeration subsystems to condenseoverhead vapors, for example from a demethanizer, a deethanizer, and/ora C₂ splitter. The temperatures employed can be in the range from about12° C. to about −100° C. These low temperatures can be achieved throughthe use of multiple refrigeration systems, such as ethylenerefrigeration and propylene refrigeration systems, to provide differentlevels of refrigeration. These can be similar to those employed inexisting steam crackers.

Alternatively, an open loop methane refrigeration system can be employedto provide refrigeration for a demethanizer OCM product effluent cancomprise methane as the major component, for example at a concentrationof at least about 50 mol %, 60 mol %, 70 mol %, 80 mol %, or 90 mol %.The demethanizer can have the lowest temperature requirements in theentire separations unit. Use of methane refrigeration (e.g.,auto-refrigeration) can provide benefits such as elimination of the needfor an additional refrigeration system (e.g., new) for any addedcapacity. For grassroots or greenfield OCM applications, this canconsiderably reduce refrigeration compressor sizes needed. In somecases, an entire refrigeration system can be eliminated. FIG. 2 shows ablock flow diagram for an exemplary open loop methane refrigerationsystem, such as can be used in gas processing plants and steam crackersto produce chilling for condensing overhead vapors from a demethanizerMost elements of FIG. 2 correspond to the description in FIG. 1; theseparations unit 205 can include an open loop methane refrigerationsystem to provide cooling for the separations. The system can becombined with a single or multiple stage (e.g., two-stage) expansionsystem (e.g., Joule Thompson) to chill the incoming feed. In certaincases, multiple separate lighter products are recovered, such as a lightH₂-rich stream, a low pressure methane rich stream, and a high pressuremethane rich stream.

Mixed Refrigeration

Another alternative to ethylene and propylene refrigeration subsystemsis the use of a mixed refrigeration system. The mixed refrigerant canbe, for example, a mix of methane, ethylene and propylene. The mixedrefrigerant can be a mix of ethane and propane. A wide range of possiblemixed refrigerants can be employed, and can be selected based on, forexample, the availability of certain components and the degree ofrefrigeration required. A mixed refrigerant system can provideadvantages for use with an OCM reactor system, including the use of onlyone refrigeration sub system. Rather than two refrigeration systems eachcomprising multiple stages of refrigerant compressor, associatedvessels, exchangers, and other components, the process can use a singlerefrigeration system. This can substantially reduce capital cost. Thiscan also reduce equipment count, which can be a benefit especially forOCM retrofits at places where plot space may be a concern.

Pi Complexation

Pi complexation techniques can be used to separate alkenes from alkanes.Some metal ions complex selectively with unsaturated organic compounds.Some of these complexes are reversible while others are irreversible.For example, aqueous silver salt in solution forms reversible complexeswith olefins, and forms irreversible complexes with acetylenes. Thisproperty can be employed in an OCM process to recover ethylene andpropylene from OCM reactor effluent.

As shown in FIG. 3, separation of ethylene and/or propylene by metalcomplexation can be divided into three major sections: absorption,purification or venting of impurities, and desorption. An exemplaryprocess is provided for separation of ethylene and/or propylene from apurified multi-component gas stream from the OCM reactor. FIG. 3 shows aprocess for purifying a stream containing ethylene using an aqueoussilver nitrate solution. Metal complexation (e.g., silver or cuprous ioncomplexation) can be used to separate ethylene and/or propylene from apurified multi-component gas stream produced via OCM comprising C₂compounds, C₃ compounds, and lighter components such as hydrogen andnitrogen. First, the multi-component gas stream 310 can be introducedinto an absorber 301 with aqueous silver salt solution, such that theethylene and/or propylene undergo absorption or complexing with thesilver metal ions, and such that trace acetylenes react with the silvermetal ions. Vent gas 311 can be removed from the absorber. Then, thesilver salt solution stream 312 can be vented 313 in a vent column 302at reduced pressure to remove any dissolved low molecular weightcomponents. Then, the resulting silver salt solution stream can betreated in a stripper 303 to separate the absorbed or complexed ethyleneand/or propylene from the silver salt solution, and further treated in atreatment unit 304 to release the trace acetylenes. Purified ethylene316 can be recovered, and some product can be recycled 317. The aqueoussilver salt stream 318 can then be recycled to the first step, in somecases after regeneration in a regeneration unit 305 with AgMnO₄ 320.MnO₂ 321 can be removed from the regeneration unit. H₂O₂ 319 can beadded to the solvent stream being returned to the absorber.

Useful adsorbents include but are not limited to metal compounds, suchas silver or copper, supported on high surface area carriers with aplurality of pores. These adsorbents can be used in pressure swingadsorption or temperature swing adsorption processes. When operatingpressure and/or temperature is changed, the silver or copper compoundcan release the alkene-rich component from the adsorbent. Theseadsorbents can be very effective for selective adsorption of alkenessuch as ethylene, propylene, and mixtures of these from gaseousmixtures.

When a gaseous component solubilizes in a liquid and complexes with itsions, the loading of the gas can be affected by its partial pressure andthe temperature and the concentration of the complexing ions in thesolution. Therefore, by changing the physical conditions separately orcollectively, the active gaseous component can either be formed into orout of the solution. Adjusting or swinging one or more physicalparameters can be used to carry out an ethylene or propylene separationusing an aqueous silver nitrate solution. Purification or venting ofimpurities can result in a product stream that is free or substantiallyfree of impurities including but not limited to CO₂, sulfur compounds,acetylenes, and hydrogen. Acetylene and hydrogen can cause operationalproblems and so the process gas can be treated to bring theconcentration of such impurities to within an acceptable limit.

Metal complexation can be used in combination with other processes, suchas membrane based processes.

Membranes

Membranes can be used to perform a variety of separations, such asseparations of olefins and paraffins, or separations of CO₂. A membranecan be essentially a barrier that separates two phases and restrictstransport of various chemicals in a selective manner. Polymer membranescan be used to separate mixtures such as propylene/propane mixtures andethylene/butene mixtures. Separations in polymeric membranes aredependent on the solubility and diffusion of the species through themembrane. While zeolite-based separations are predominantly depended onmolecular size differences, the differing permeation of olefins througha polymeric membrane can be largely attributed to differences insolubility, which can depend on the critical temperature and the kineticdiameter. Membrane separations can be employed even when there are smallmolecular size differences.

The OCM process can utilize a membrane based separation process tofurther enhance the efficiency and energy consumption of the process.Cryogenic distillation can be used for the separation of alkenes, but ishighly energy intensive. Membrane based separations can be used for avariety of purposes in the context of an OCM process, such as toseparate and purify ethylene product from OCM reactor effluent, toseparate a stream rich in CO₂, to separate a stream containing lighterhydrocarbons and inerts, or to separate C₂ compounds from C₁ and lightercompounds.

Membranes can include but are not limited to isotropic membranes,anisotropic membranes, and electrically charged membranes. A membranecan be a ceramic membrane, a metal membrane, or a liquid membrane. Anisotropic membrane can be a microporous membrane or a non-porous densemembrane. Membranes can be used for separations including but notlimited to CO₂ separation, paraffin-olefin separation, or selectiverecovery of pure ethylene from the OCM reactor effluent. Polymer derivedcarbon molecular sieve membranes can be used to separate paraffins fromolefins. These membranes can be used, for example, to separate ethylenefrom a mix of methane and ethane.

Membrane separations can be used in combination with other types ofseparation and purification subsystems to remove other impurities suchas acid gases, hydrogen, and nitrogen.

Transport through a membrane can take place when a driving force isapplied to the components in the feed. A driving force can be a pressuredifferential or a concentration (activity) gradient across the membrane.Membrane based separation techniques can be used in an OCM process byapplying either of the above mentioned driving forces. A membrane basedseparation can also be a component of a hybrid separation set-up, suchas a membrane and an absorption system (e.g., a membrane contactor) or amembrane in a pressure swing adsorption (PSA) or a temperature swingadsorption (TSA) system.

An OCM reactor can employ membranes as a part of the reactor system toeffectively separate the ethylene product within the reactor systemitself. A section of the reactor can include membranes that aid inrecovering the ethylene product, with a methane rich stream beingrecycled to a methanation system and eventually to the OCM reactor. Sucha system can also use advanced heat recovery or quench methods so as tofacilitate the use of membranes.

Pressure Swing Adsorption (PSA) and Adsorption Technology

Cryogenic separation (e.g., distillation) can be used for the recoveryof ethylene, propylene, and other components from olefin plants,refinery gas streams, and other sources. These separations can bedifficult to accomplish because of the close relative volatilities, andcan have significant temperature and pressure requirements foroperation. The ethane/ethylene distillation can be performed at about−25° C. and 320 pounds per square inch gauge (psig) in a columncontaining over 100 trays. Distillation of propane and propylene can beperformed at about −30° C. and 30 psig. These can be some of the mostenergy intensive distillations in the chemical and petrochemicalindustry. In general, the use of distillation towers to separate recoverand purify components is an energy intensive process.

The present disclosure provides the use of adsorbents that can achieveseparation and purification of olefin rich streams. In particular, thepresent disclosure applies the use of PSA-based adsorbent systems toseparate, purify, and recover olefins like ethylene and propylene fromstreams containing one or more impurities such as methane, hydrogen,carbon monoxide, carbon dioxide, ethane, or others. The streams, orparts of the streams, can be generated via an OCM process, an ETLprocess, or combinations thereof. The streams can be final productstreams where PSA is used to recover and purify the final product. Thestreams can be intermediate streams which are purified prior to use as afeed in a subsequent process, such as an ETL process, an ethylenecracker (steam cracker), a refining unit, a fuel gas system, a naturalgas recovery plant or any other product fractionation or producttreatment unit.

Pressure Swing Adsorption (PSA)

A pressure swing adsorption (PSA) process cycle is one in whichdesorption takes place at a different (e.g., lower) pressure than theadsorption pressure. Reduction of pressure can be used to shift theadsorption equilibrium and affect regeneration of the adsorbent. Lowpressure may not be as effective as temperature elevation in totallyreversing adsorption, unless very high feed to purge pressure ratios areapplied. Therefore, most PSA cycles are characterized by high residualloadings and thus low operating loadings. These low capacities at highconcentration require that cycle times be short for reasonably sizedbeds (e.g., seconds to minutes). These short cycle times are attainablebecause particles of adsorbent respond quickly to changes in pressure.Major uses for PSA processes include purification as well asapplications where contaminants are present at high concentrations.

As shown in FIG. 4, the PSA system can comprise two fixed bed adsorbers401 and 402 undergoing a cyclic operation of four steps—adsorption,blowdown, purge, and pressurization. The PSA system can receive a feed410 and produce a product stream 411, with a PSA off gas stream 412. Forimproving the performance of the basic Skarstrom™ cycle (FIG. 4),additional operation steps can be employed such as pressureequalization, product pressurization, and co-current depressurization.Besides these steps, the number of beds can be modified to achieve theoptimal operation and multi-bed processes can be used in commercialapplications like hydrogen recovery. Similarly, a TSA system can be usedwhere a swing in temperature causes the sorption and desorption.

PSA cycles are used primarily for purification of wet gases and ofhydrogen. High pressure hydrogen employed in processes such ashydrogenation, hydrocracking, and ammonia and methanol production can beproduced by PSA beds compounded of activated carbon, zeolites and carbonmolecular sieves. Other exemplary applications include: air separation,methane enrichment, iso/normal separations, and recovery of CO and CO₂.

Adsorbents

Adsorbents can be natural or synthetic materials, such as those havingamorphous or microcrystalline structure. Exemplary adsorbents useful forlarge scale operation include but are not limited to activated carbon,molecular sieves, silica gels, and activated alumina. Other usefuladsorbents include pi complexation sorbents, silver and coppercomplexation adsorbents, zeolites, synthetic zeolites, mesoporousmaterials, activated carbons, high surface area coordination polymers,molecular sieves, carbon molecular sieves (CMS), silica gels, MCM,activated alumina, carbon nanotubes, pillared clays, and polymericresins.

For systems where the incoming stream is a multi-component mixture ofgases and the number of compounds to be separated cannot be removed by asingle adsorbent, different layers of adsorbents can be used. Forexample, hydrogen purification from a methane stream in a reformingoperation, where H₂ is contaminated with H₂O, CO₂, CO, and unconvertedCH₄, can employ activated carbon to remove H₂O and CO₂ in combinationwith additional layers of different adsorbents used to increase theloading of CO.

Zeolites, molecular sieves, and carbon molecular sieves (CMS) can beused for most industrial separations employing PSA. Inorganic materials,like special kinds of titanosilicates, can be used for kineticseparations.

For systems specifically configured to separate ethane/ethylene andpropane/propylene, exemplary types of adsorbents includezeolites/molecular sieves and pi complexation sorbents.Zeolites/molecular sieves can be used for kinetic separation, such asseparation based on higher diffusivity of olefins over that ofparaffins. The use of 4 A zeolite is one such example. Pi complexationsorbents, such as AgNO₃/SiO₂, can give excellent results as compared to4 A zeolite. PSA units as discussed herein can employ a range ofdifferent sorbents, including but not limited to a zeolite/molecularsieve sorbent, a pi complexation based sorbent, a carbon molecular sievesorbent or any other form of activated carbon, carbon nanotubes,polymeric resin based sorbents, or other sorbents.

Adsorbents can be selected based on a number of different criteria.Adsorbent selection criteria can include capacity for the targetcomponents (e.g., affinity for the desired components to be separatedfrom the multi-component feed stream), selectivity between componentscompeting for same adsorption sites, regenerability of the adsorbent,(e.g., the ability of the adsorbent to release the adsorbed targetcomponents at a reasonable pressure rate of gas diffusion into theadsorbent—this can also affect the size of the bead that is chosen andconsequently the pressure drop across the bed; an insufficient diffusionrate can require smaller diameter beads that can result in higherpressure drop and hence increased operating costs), and chemicalcompatibility (e.g., selecting an adsorbent resistant to chemical attackthat may poison or destroy the adsorbent, such as liquid hydrocarbonscausing physical breakdown of the adsorbent resulting in loss ofefficiency and back pressure).

CO₂ Separation

There are many technologies available for CO₂ capture, such as from fluegases, natural gas, or from any process gas rich in CO₂. Variousprocesses for post-combustion or pre-combustion capture can be usedreduce CO₂ emissions. FIG. 5A, FIG. 5B, and FIG. 5C show exemplaryschematics of different separation methods available to separate CO₂from a process gas or a flue gas.

OCM processes can utilize an amine based absorption system for CO₂removal, which can be followed by use of a caustic scrubber to obtainhigh degree of separation. The amine system is prone to corrosion,solvent degradation, and above all, has high energy requirements.Separations with sorbents and/or solvents can involve placing the CO₂containing gas in intimate contact with a liquid absorbent or a solidsorbent that is capable of capturing the CO₂. As shown in FIG. 5A, astream with CO₂ 510 can be directed into a capture vessel 501, where itcontacts sorbent which captures CO₂ from the stream. The stream, withreduced or removed CO₂, can then exit 511 the vessel. Sorbent 512 loadedwith captured CO₂ can be transferred to a sorbent regeneration vessel502 where it releases the CO₂ after being heated (e.g., with the use ofenergy 513), after a pressure decrease, or after any other change in theconditions around the sorbent, thereby regenerating the sorbent. Spentsorbent 515 and CO₂ 516 can be removed from the vessel, and make upsorbent 513 can be added. After the regeneration step the sorbent can besent back to capture more CO₂ in a cyclic process. The sorbent can be asolid. Solid sorbent can remain in a single vessel rather than beingcycled between vessels; sorption and regeneration can be achieved bycyclic changes (e.g., in pressure or temperature) in the vessel wherethe sorbent is contained. A make-up flow of fresh sorbent can be used tocompensate for natural loss of activity and/or sorbent losses.

Amine scrubbing technology can be used to remove acid gases from processgases. Primary amines (e.g., MEA, DGA), secondary amines (e.g., DEA,DIPA), tertiary (e.g., MDEA, TEA), sterically hindered amines, chilledammonia, potassium carbonate, and other compounds can be used to removeCO₂ from process gases. Traditional amine based systems can becharacterized by high energy requirements and solvent degradation.Improved solvents, which can require less energy for regeneration of thesolution, include the Benfield process and two stage diethanolamine.Combination with an OCM process can reduce the energy consumption ofamine scrubbing processes. Improved solvents can reduce the energyrequirements by as much as 40% compared to the traditional MEA solvents.This has the potential of reducing the energy, and hence steam,consumption of the OCM process, thereby increasing the amount of steamavailable for export from the OCM, or making alternative waste heatrecovery methods feasible.

Physical absorption solvents used can include but are not limited toglycol dimethylethers (e.g., Selexol) and propylene carbonate (e.g.,IPTS/EC). Regeneration of the solution can be performed by vacuumflashing and air stripping; this approach can consume significantly lessenergy than in chemical absorption. In using physical solvents CO₂ canbe released mainly by depressurization, thereby avoiding the high heatof consumption of amine scrubbing processes.

Mixed or hybrid solvents can include but are not limited to Sulfinol™(sulfolane, water, and amine), such as Sulfinol-M and Sulfinol-X.

Solid adsorbents, such as zeolites and activated carbon, can be used toseparate CO₂ from gas mixtures. In pressure swing adsorption (PSA), agas mixture can flow through a packed bed of adsorbent at elevatedpressure until the concentration of the desired gas approachesequilibrium. The bed can be regenerated by reducing the pressure. Intemperature swing adsorption (TSA), the adsorbent can be regenerated byraising its temperature. In general usage, adsorption is not yetconsidered attractive for large scale separation of CO₂ because thecapacity and CO₂ selectivity of available adsorbents are low. However,when the OCM process is a recycle process, an adsorbent based separationmethod can be used to separate bulk CO₂ followed by consuming theremaining CO₂ in a methanation reactor system, or by using a causticscrubber to treat the remaining CO₂.

Many different types of membrane materials (e.g., polymeric, metallic,ceramic) can be used for CO₂ capture to preferentially separate CO₂ froma range of process streams. FIG. 5B shows an exemplary schematic ofseparation of CO₂ from a gas stream 530 in a separation vessel 520 usinga membrane 521. CO₂ can be removed from the stream via the membrane, andCO₂ and other gases can exit the vessel in separate streams 531 and 532.The main limitation of currently existing membranes is the occurrence ofsevere plasticization of the membrane in the presence of high pressureCO₂. Due to excessive swelling of the polymer membrane upon exposure toCO₂, the performance (e.g., selectivity) can decrease significantly,thus reducing the purity of the CO₂ and consequently reducing thepossibilities for reuse of the gas. Energy requirements can besignificantly lower for membrane based technologies; for example,membrane technology can use 70-75 kWh per ton of recovered CO₂ comparedto significantly higher values for pressure swing adsorption (e.g.,160-180 kWh), cryogenic distillation (e.g., 600-800 kWh), or amineabsorption (e.g., 330-340 kWh), making membrane technology an attractiveoption for integration with OCM for CO₂ separation.

Membrane and amine technologies can be combined to form a hybrid processto capture CO₂. Micro-porous hollow fiber membranes can be used for CO₂separation using amine-based chemical absorption processes. Micro-porousmembranes can be used in a gas-liquid unit where the amine solution iscontacted with CO₂ containing gas. Using the membrane can lead to areduction in the physical size and weight of the gas-liquid contactingunit. The separation is based on reversible chemical reaction, and masstransfer occurs by diffusion of the gas through the gas/liquid interfaceas in traditional contacting columns. Such a hybrid membrane contactorcan provide a high contact area between gas and liquid, reduce oressentially eliminate foaming and flooding problems, and give betteroperational flexibility while reducing solvent degradation problems.

A membrane contactor can combine the advantages of membrane technologyand solvent absorption for CO₂ separation. A membrane contactor is acombination of advanced membrane techniques with an effective absorptionprocess. A membrane contactor is a hybrid mass exchanger where a porousmembrane separates two phases. The selective sorbent performs theseparation while the membrane facilitates the mass exchange process byexpanding the phase contact surface area. The modified surfaceproperties can improve the selectivity of the process by selectivelyinhibiting the transport of one of the mixture constituents. Compared toa conventional column device, membranes can allow for up to five timesincrease in yield per unit volume. Since the sorptive liquid flowswithin capillaries and both phases are not directly contacting eachother, membrane absorbers can operate in any spatial configuration(horizontal or vertical) and at any flux rations between both phases.Also, there is no flooding or uneven packing moisturization. Since thesystem operates with unchanging yields, independent of the diameter andheight; scaling up is fairly simple. Membranes used can bemicromembranes or ultrafiltration membranes made a variety of differentpolymer and ceramic materials. Polypropylene fiber membranes can be usedto separate CO₂ from CH₄, for example by using amines like MEA asabsorption liquid. Hollow fiber membranes, such as porous polypropylene,perfluoroalkoxy (PFS), and asymmetric poly(phenylene oxide) hollow fibermembranes with a dense ultrathin skin at the outside of the membrane canalso be used. Besides amines as absorption liquid, other absorptionliquids may be used, such as aqueous sarcosine salt solutions, forexample in a gas-liquid membrane contactor system. A membrane contactorcan be used to separate the CO₂ from the OCM effluent in which CH₄ isthe major component. Membrane contactors can also be used for separationof olefins and paraffins, and the separation of CO₂ from light gases.

An activator, such as piperazine, diethanolamine, and arsenic trioxide,can be used to further enhance the effectiveness of CO₂ capture. DGA andtertiary amines may provide more improvement than primary or secondaryamines.

Gas selective poly ionic liquid membranes, which are polymerized roomtemperature ionic liquids (RTIL), can be used to be highly selectivelyseparate CO₂. RTILs can be synthesized as a monomer and subsequentlypolymerized to obtain gas selective membranes. The ionic nature of thepolymers can result in tight arrangements between the oppositely chargedionic domains in the poly RTIL, which can eventually prevent themembrane from excessive swelling and deterioration of its performance atincreased pressure and/or temperature. This intrinsic property of polyRTIL can be used to increase the resistance against plasticization andto restrict strong swelling of the polymer membrane to maintain itspermeation properties in the presence of a strong plasticizing agentsuch as CO₂ at higher pressures. For example, an imidazolium-based polyRTIL can be used as base material and the length of the alkyl chain canserves to strengthen or weaken the ionic interactions within the polyRTIL. High pressure mixed CO₂/CH₄ gas separation measurements atdifferent temperatures.

Gas components like CO₂, from N₂ or CH₄ can be separated with supportedionic liquid membranes. Ionic liquids are molten salts with a very lowmelting point (many are liquids at room temperature). Many ionic liquidsshow a high solubility for carbon dioxide and hence can be highlysuitable for use with an OCM process. For example, ionic liquids caninclude but are not limited to imidazolium, pyrollidinium, pyridinium,cuanidinium, phosphonium, morpholinium, piperidinium, sulfonium,ammonium, hexafluorophosphate, tetraflouroborate, alkylsulphate,triflate, dicyanamide, bis(trifluoromethylsulfonyl)imide, andcombinations thereof. Specific advantages of ionic liquids include verylow to negligible vapor pressure, good dissolution characteristics formany substances, and lack of flammability or toxicity. Ionic liquids canhave good thermal, mechanical and chemical stability as well asfavorable densities and viscosities. The required specifications can beadjusted easily by the large number of possible combinations of anionsand cations when formulating an ionic liquid. Ionic liquids can be usedas chemical solvents, catalysts, electrolytes in fuel cells as well asfor gas-separation and storage by absorption. Ionic liquid membranesystems can comprise an adequate porous support material, e.g. a polymerfilm, coated by ionic liquids. The system separated CO₂ and sulfurcompounds from different gas mixtures. Competitive selectivity andpermeability are obtained for the separations.

Novel membrane materials, such as polyetherimides, can be used asmembrane material with improved plasticization resistance for CO₂removal, for example with an OCM process. Other membrane materials thatcan be used include polymeric membranes based on polyamides,polysemicarbazides, polycarbonates, polyarylates, polyaniline,poly(phenylen oxide), polysulfones, and polypyrrolones. In some cases,the polymeric membrane is solvent resistant and can reduce theplasticization effects of hydrocarbons in the feed stream, e.g.,polyketone, polyether ketone, polyarylene ether ketone, polyimide,polyetherimide, and polyphenylene sulphide, which have intrinsic solventinertness and can therefore withstand organic rich operation conditions.

An adequate porous support material, e.g. a polymer film, coated byionic liquids can be used in continuous separation of CO₂ and sulfurcompounds from different gas mixtures, including a methane rich stream.This separation can improve the efficiency of OCM processes. The OCMreactor effluent can enter the supported ionic liquid separationsubsystem, and CO₂ and other contaminants can be removed from theprocess gas. Other contaminants can include but are not limited totraces of sulfur compounds, inerts, CO, SO₂, H₂S, andtetrahydrothiophene (THT).

CO₂ can be separated from other gases by cooling and condensation, forexample as shown in FIG. 5C. A stream containing CO₂ 550 can becompressed in a compressor 540, and the compressed stream 551 can bedirected to a distillation column 541. Some components can be recoveredfrom the overhead stream 552, with heat recovered in a heat exchanger542. Other components can be recovered from the bottoms 555. Cryogenicseparation is widely used commercially for streams that already have ahigh concentration of CO₂ (typically greater than 90%). Cryogenicseparation of CO₂ has the advantage that it enables direct production ofhigh purity liquid CO₂ that can be used as a feedstock to convert thecarbon to higher value hydrocarbons, or otherwise be captured. Theamount of energy required can be high, and water may need to be removedbefore the feed gas is cooled.

Low temperature distillation can give better results when there is ahigh concentration of CO₂ in the feed gas. For the OCM process gas, theCO₂ concentration can be increased by, for example, having a recyclestream, or by using a modified OCM reactor where excess CO₂ is used as aquench medium for the reaction heat. Low temperature separation canrefer to separations using temperature levels above −90° C.

FIG. 6 shows a schematic of CO₂ separation using distillation. OCMreactor effluent 606 can be fed to a treatment unit 601, such as amolecular sieve dryer, a sulfur removal bed, or an acetylene removalbed. The treated gas is fed to the first distillation column 602 thatseparates the bulk of the methane from the CO₂ and other heavierhydrocarbons. Depending on the CO₂ concentration in the stream 606, thebottom stream 608 may contain 50%, 60%, 70%, 80%, 90% (or anywhere inbetween) of the incoming CO₂. The overhead from 607 contains majority ofthe methane and other light gases and is fed to the column 603. Column603 further recovers methane rich gas 611, which can be the feed to amethanation system. The bottoms product 616 may be recycled or sent as apurge to the fuel gas system. The CO₂ rich gas 608 is distilled in theCO₂ column 604 to recover pure CO₂ 609 in the overhead. The bottomsproduct 610 can contain some methane along with ethane, ethylene, andother heavier hydrocarbons, and can be sent to recover the ethyleneproduct in a separator 605. The CO₂ product can be sent to methanationunit, and a part of the CO₂ can be recycled to achieve the desiredconcentration of CO₂ in the feed stream 606. Such a CO₂ distillation subsystem can offer many benefits, including but not limited to reducingthe loop size of the OCM process considerably, as the function of theexisting cryogenic demethanizer can be reduced by a large extent.Additionally, amine and caustic systems can be replaced by cryogenic orlow temperature distillation systems.

Alkaline salt-based processes can be used for carbon dioxide removal.These processes can utilize the alkali salts of various weak acids, suchas sodium carbonate and potassium carbonate. These processes can provideadvantages such as low cost and minimal solvent degradation. Processesthat can be used for H₂S and CO₂ absorption include those using aqueoussolutions of sodium or potassium compounds. For example, potassiumcarbonate can absorb CO₂ at high temperatures, an advantage overamine-based solvents.

Hot potassium carbonate (K₂CO₃) solutions can be used for the removal ofCO₂ from high-pressure gas streams, among other applications. Potassiumcarbonate has a low rate of reaction. To improve CO₂ absorption, masstransfer promoters such as piperazine, diethanolamine, and arsenictrioxide can be used. Less toxic promoters such as borate can also beused, for example with flue gas streams (see, e.g., Ghosh et al.,“Absorption of carbon dioxide into aqueous potassium carbonate promotedby boric acid”, Energy Procedia, pages 1075-1081, February 2009, whichis hereby incorporated by reference in its entirety). To limitcorrosion, inhibitors can be added. These systems can be known asactivated hot potassium carbonate systems. Licensed hot activatedpotassium carbonate systems include the Benfield™ and the Catacarb™process. The processes can be used for bulk CO₂ removal fromhigh-pressure streams, but can also produce high-purity CO₂.

Flue gas impurities such as SOx and NOx can reduce the operationalefficiency of the potassium carbonate as a solvent. SO₂ and NO₂ may notable to be released from the solvent under industrial conditions.Selective precipitation of the impurity salts formed by SOx and NOx canbe used to remove such compounds (see, e.g., Smith et al., “Recentdevelopments in solvent absorption technologies at the CO2CRC inAustralia” Energy Procedia, pages 1549-1555, February 2009, which ishereby incorporated by reference in its entirety).

A variety of materials can be used as CO₂ sorbents through chemicalreactions and physical absorptions, including but not limited tosoda-lime, active carbon, zeolites, molecular sieves, alkali metaloxides, silver oxide, lithium oxide, lithium silicate, carbonates,silica gel, alumina, amine solid sorbents, metal organic frameworks andothers.

Physical impregnation of CO₂-reactive polymers, such as tetraethylenepentamine or polyethyleneimine, inside a porous support, such asalumina, pumice, clay or activated carbon, can be used for CO₂ removal.Amine based sorbents can be easily regenerated. Alternatively, a mixtureof an amine compound with a polyol compound can be impregnated in aporous support. The polyol compound can be used to increase the CO₂desorption rate of the amine. The supported amine-polyol sorbent cancomprise from about 1 wt % to about 25 wt % amine and from about 1 wt %to about 25 wt % polyol, with the balance being the support. Solidsorbent can adsorb and desorb CO₂ a relatively high rates at ambienttemperatures Enhanced CO₂ cyclic removal capacities in either dry orhumid air flows can further be achieved by using a solid sorbent at anincreased amine concentration of amines from about 35 wt % to about 75wt %.

Solid sorbents that can selectively remove multiple gases can be used toremove CO₂, H₂O, nitrogen oxides, and hydrocarbons. This can be achievedby using composite adsorbents, for example by using a mixed adsorbent ofalumina and zeolite to remove CO₂ and H₂O simultaneously.

CO₂ can be separated from flue gas using an ion pump method instead ofrelying on large temperature and pressure changes to remove CO₂ from asolvent. Ion pump methods can dramatically increase the overlying vaporpressure of CO₂. As a result, the CO₂ can be removed from the downstreamside of the ion pump as a pure gas. The ion pumping can be obtained fromtechniques including but not limited to reverse osmosis, electrodialysis, thermal desalination methods, or an ion pump system having anoscillation flow in synchronization with an induced electric field.

By making use of energy such as renewable or nuclear energy, carbondioxide and water can be recycled into sustainable hydrocarbon fuels ina non-biological process. Various pathways can enable such a conversion,for example by H₂O and CO₂ dissociation followed by fuel synthesis. Themethods of dissociation can include heat, electricity, and solar drivenmethods such as thermolysis, thermochemical loops, electrolysis, andphotoelectrolysis. High temperature electrolysis can make efficient useof electricity and heat, provide high reaction rates, and integrate wellwith fuel synthesis.

Synthetic analogues of enzymes as a polymer thin film supported onmicro-porous substrates can be used to separate CO₂ from gas mixtures.For example, a polymer thin film containing carbonic anhydrase mimickingsites can supported on a porous substrate and can separate CO₂ from astream containing O₂ and N₂. The system can be, for example, about 30%lower in cost compared to amine-based systems.

Process Configurations Electrolysis to Generate Oxygen and Hydrogen forOCM Process

Electrolysis can be used to produce industrial hydrogen. OCM processescan have a lot of synergistic benefit from deploying a waterelectrolysis subsystem with the OCM process. The water electrolysis unitcan replace an air separation unit (ASU) to supply the oxygen requiredfor the OCM process. The products from the electrolytic unit can beconsumed within the OCM process: oxygen can be consumed within the OCMreactor and hydrogen can be used in a methanation reactor. Availabilityof more hydrogen in the methanation unit has the potential to increasethe carbon efficiency to about 100%, by converting the CO₂ produced inthe OCM reaction to methane, which can be recycled back to the OCMreactor. The OCM unit can be a net exporter of high purity excesshydrogen, after consuming the entirety of the CO₂ produced in the OCMProcess.

The water electrolysis subsystem can be an electrolytic cell employingalkaline water electrolysis, a proton exchange membrane electrolysissystem, or a steam electrolysis system. The electricity source to theelectrolytic sub system can be renewable, such as photo voltaic/solarpower, which can make the entire system 100% carbon efficient with azero carbon footprint. A storage system for oxygen, or a backup powersupply, may be used to ensure the continuous supply of oxygen andhydrogen.

With steam electrolysis, a substantial part of the energy needed for theelectrolysis process can be added as heat, which can be much cheaperthan electric energy, and which the OCM reactor can produce inabundance. Therefore, integration of steam electrolysis can takeadvantage of the extra heat from the OCM reactor to provide energy forthe steam electrolysis. This can be of particular benefit to OCMdeployments where no additional steam or power is required.

FIG. 7 depicts an exemplary electrolysis subsystem combined with an OCMsystem. The electrolysis subsystem 701 can take water 710 and electricpower 711 as inputs and generate pure oxygen 712 and hydrogen 713 asproducts. The oxygen can be fed into an OCM reactor 702 with a methanefeed 714, for conversion to higher hydrocarbon products includingethylene. The OCM product stream can be compressed in a compressor 704and separated in a separations unit 705. Higher hydrocarbon products 716can be recovered from the separations unit, and other compounds such asmethane and CO₂ can be recycled 717 and/or purged 718. The recyclestream can be directed to a methanation unit 703, which can generatemethane 715 using the hydrogen from the electrolysis subsystem. Theextra hydrogen that is now available to the methanation unit can enablethe conversion of most or all of the CO₂ produced in the OCM process tomethane, which can drive the process to a higher efficiency. The processcan also be almost 100% emission free. The CO₂ produced in the processthat may be discarded as waste may be converted to methane and hence toethylene in the OCM reactor.

Different Quench Media for the OCM Reaction

The OCM reaction is highly exothermic. Various quenching media can beused to extract the OCM reaction heat. For example, CO₂ can be injectedto extract the heat, which results in the OCM effluent containing excessCO₂; such effluent can be suitable for the advanced CO₂ recovery methodsdescribed herein. FIG. 8 shows an exemplary system where CO₂ 814 isremoved from an OCM product stream 812 (generated in an OCM unit 801from an oxygen stream 810 and a methane stream 811) in a CO₂ separationunit 802 and recycled from back to the OCM reactor 801. A waste gas orpurge stream 815 can also be removed from the CO₂ separation unit. TheOCM product stream 813 can then be separated in a separations unit 803into a product stream 816 comprising ethylene and a purge and/or recyclestream 817. Separation methods can include low temperature separation,membrane separation, or other separation methods discussed herein. TheOCM loop can be decreased to just a CO₂ recycle stream. The system canalso comprise a methanation unit (not shown).

Such an approach can provide advantages including a smaller recycle loopand more efficient CO₂ removal methods, resulting in lower capitalexpenditure (CAPEX). This can also result in the feasibility of smalldistributed scale OCM units, since after the removal of excess CO₂, therelatively richer ethylene stream needs fewer treatment and recoverysteps.

Heat Recovery

Waste heat from the OCM process can be used to generate superheated highpressure steam that can be used in the process, exported to other userson site, or can be used to generate power. Excess process heat can alsobe used to preheat the feed streams. Other uses for excess heat can beless capital intensive, and offer a greater operational flexibility andlow maintenance. Thermoelectric energy conversion can be used to convertwaste heat to power. Example uses for waste heat include single fluidrankine cycles (e.g., steam cycle, hydrocarbons, and ammonia),binary/mixed fluid cycles (e.g., ammonia/water or mixed hydrocarboncycle).

Organic Rankine Cycle

The organic Rankine cycle (ORC) can be used to generate power from heat.In ORC, an organic component is used instead of water. The organiccompound can be a refrigerant, a hydrocarbon (e.g., butane, pentane,hexane), silicon oil, or a perfluorocarbon. The boiling point of theorganic fluid can be lower than that of water, which can allowrecovering heat at a lower temperature than in the traditional steamRankine cycle.

Owing to the exothermicity of the OCM reaction, the ORC system can bedeployed as a waste heat recovery method for use with OCM. Waste heat atrelatively low temperature can be recovered by an intermediate heattransfer loop and used to evaporate the working fluid of the ORC.

FIG. 9 shows an exemplary OCM system with an ORC subsystem. The workingfluid can be chosen which can be condensed with cooling water or air atnormal atmospheric pressure. FIG. 9 shows the heat source as the OCMreaction heat from an OCM unit 901. Heat can be recovered from the OCMproduct stream 910 in an evaporator 902, and the product stream 911 canthen be directed for downstream processing from the OCM unit. The heatrecovered in the evaporator can be used to evaporate a working fluidstream 912, which can then be directed to a turbine 903 to generatepower in a generator 904. From the turbine, the working fluid 913 can bedirected to a condenser 905 and cooled using a cooling medium 914. Thecooled working fluid 915 can then be pumped by a pump 906 in a stream916 back to the evaporator.

Thermoelectric Power Generation

The OCM process can make use of a heat exchanger with thermoelectric(TE) generators for heat recovery. A Thermoelectric Power Generator(TPG) can have four basic components: Heat source, P and N typesemiconductor stack (or a TE module), heat sink (cold side), and anelectrical load (output voltage). The TE module can include two or moreof P-type and N-type semiconductor pellets connected in series orparallel depending on the served load.

The TE devices can be solid state engines that do not require anyworking fluid. Thermoelectric materials can provide efficiencies of upto 15% or greater. Thermoelectric generators coupled with heatexchangers can produce electricity even at temperatures as low as 350 Kwith low maintenance. TE modules can be used with OCM including largebulk TE modules and thin film or micro TE modules.

For high temperatures, micro TE modules can be used. Micro TE modulescan also have low equipment weights. TE devices can be very reliable,scalable, and modular. Some TE modules can give best results at smallscales. The OCM process can generate medium level waste heat that ishighly suitable for a TE device to generate power.

OCM and ETL Systems with Advanced Separations Sub-Systems

PSA technology can be applied to processes including those involving ahydrocarbon stream containing a mix of the following hydrogen, carbondioxide, carbon monoxide, methane, ethane, ethylene, propane, propylene,butanes, butenes and/or other higher hydrocarbons needing to be purifiedor separated into desirable products (e.g., ethylene, methane, hydrogen,or propylene).

Hydrocarbon streams can be produced via traditional refining andpetrochemical processes. Hydrocarbon streams can be produced from OCM orETL reactor systems.

The present disclosure provides the use of PSA in processes and systemsfor oxidative coupling of methane (OCM) and ethylene-to-liquids (ETL)operations, and the application of adsorbent based processes used inconjunction with OCM and ETL processes to generate significant processimprovements and enhance the economic value of the processes. OCMsystems are described in, for example, U.S. patent application Ser. No.14/592,668, which is entirely incorporated herein by reference. ETLsystems are described in, for example, U.S. patent application Ser. No.14/591,850, which is entirely incorporated herein by reference.

An OCM system, such as that shown in FIG. 10, can include an OCM orOCM-post-bed-cracking (PBC) reactor 1002, a process gas compressionsystem 1003, a process gas treatment system 1004, a cryogenicseparations system, and a methanation system 1001. The feed to the OCMsystem can be an oxygen feed 1012 and a methane source feed 1011 (suchas a natural gas feed stream or other methane source). In some cases,additional ethane feed can be supplied to the PBC section of the OCMreactor, where paraffins such as ethane in the OCM product stream and/oradditional ethane can be cracked to olefins such as ethylene. Theseparations sub-system can comprise a series of fractionation towers,like a demethanizer 1005, deethanizer 1006, C₂ splitter 1007,depropanizer 1008, debutanizer, and others. Overhead 1013 from thedemethanizer can be directed into the methanation system along withhydrogen or natural gas 1010 to produce additional methane. The bottomsstream 1014 from the demethanizer can be directed to the deethanizer.The overhead stream 1015 from the deethanizer can be directed to the C₂splitter, and there split into ethylene 1016 and ethane 1017 streams.The bottoms stream 1018 from the deethanizer can be directed to thedepropanizer, and there split into a C₃ product stream 1019 and a C₄₊product stream 1020. The cryogenic separations system can compriseadditional ethylene and propylene refrigeration sub-systems to providefor the chilling requirements of the system.

OCM process standalone with advanced separations systems

In certain cases, the separations section of the OCM system can beeliminated, or partially eliminated, by utilizing an advancedseparations method as discussed in this application. The advancedseparation method can be a PSA unit or a membrane based method, or acryogenic system. FIG. 11 shows an exemplary schematic of OCM with a PSAunit. The PSA unit can separate methane, CO₂, CO, and/or H₂ from ethane,ethylene, propane, propylene, and/or higher hydrocarbons. Methane 1111and oxygen 1112 can be directed into an OCM reactor 1102 and reacted toproduce higher hydrocarbon products including ethylene. The OCM productcan be compressed in a process gas compression system 1103, treated in aprocess gas treatment system 1104, and separated in the PSA 1105 into aproduct stream 1113 and a recycle stream 1114. The recycle stream can bedirected to a methanation unit 1101, which can also receive a naturalgas stream 1110 and produce methane for the OCM reactor. The extent ofseparation and degree of recovery can depend on the type ofadsorbent(s), pressure differential, and number of PSA stages employed.The feed to the PSA unit can have one or more of the followingcomponents: H₂, N₂, O₂, CO, CO₂, CH₄, ethane, ethylene, acetylene,propane, propylene, butanes, butenes, butadiene, water, and higherparaffinic and olefinic components. The PSA product gas can comprisecomponents including but not limited to: H₂, N₂, CO, CO₂, CH₄, O₂,ethane, ethylene and acetylene. PSA product gas can comprise componentsfrom 0% to 99.99% recovery. The PSA tail gas can comprise 99.99%, 90%,80%, 70%, 60%, 50% ethylene. The PSA tail gas can comprise at least99.99%, 90%, 80%, 70%, 60%, 50% ethylene. The PSA tail gas can compriseabout 99%, 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, 10%, 0% ethane. ThePSA tail gas can comprise at least about 99%, 90%, 80%, 70%, 60%, 50%,40%, 30%, 20%, 10%, 0% ethane. The PSA tail gas can comprise about 60%,50%, 40%, 30%, 20%, 10%, 0% methane, hydrogen, acetylene, N₂, O₂, H₂O orCO₂. The PSA tail gas can comprise at least about 60%, 50%, 40%, 30%,20%, 10%, 0% methane, hydrogen, acetylene, N₂, O₂, H₂O or CO₂. Based onthe process configuration, including the type of adsorbents employed,pressure differential and the operation, various different recoveriesare possible.

As discussed above, the PSA unit can comprise one or more adsorbentmaterials that can be suitable to achieve the component recoveries. Thesorbent can be a zeolite/molecular sieve based material, a carbon basedsorbent, or a π-complexation sorbent. In some cases the sorbent materialcan be a polymeric resin, carbon nanotubes, and carbon fibers. The PSAunit can be configured to have layers of different sorbents so as toresult in high recoveries from the multi-component feed streams to thedesired products.

In certain cases the PSA can be a multi stage unit (see, e.g., FIG. 12).In such a unit, an OCM reactor 1202 can receive a methane stream 1211and an oxygen stream 1212, and react the methane and oxygen to producehigher hydrocarbon products including ethylene in an OCM product stream.The OCM product stream can be compressed in a first compressor 1203 anddirected to a first PSA separation 1204. The tail gas 1214 from thefirst PSA can be compressed in a second compressor 1205 and fed to asecond PSA separation 1206, the tail gas 1216 from which can becompressed in a third compressor 1207 and separated in a third PSAseparation 1208. The tail gas from the third PSA can be the finalpurified stream 1217 containing ethylene up to 99.9% purity. PSA productstreams 1213, 1215, and 1218 can be directed to recycle, such as via amethanation unit 1201 along with a natural gas stream 1210. Each PSAstage can be a dual-bed PSA or a multi-bed PSA system.

In certain cases, the process requirements can dictate that only alimited amount of recovery is required in the PSA unit and subsequentrecovery and purification is performed in a fractionation column or thegas is a feed for a downstream process unit. The downstream process unitcan be an ETL system, an ethylene steam cracker system, a gas processingplant, NGL extraction plant, a refinery off-gas separations system, orother process unit.

Retrofits for OCM

OCM can be employed to convert a feedstock comprising methane toethylene and other olefins. Historically, ethylene has been produced viasteam cracking of gaseous or liquid hydrocarbon feedstocks like ethane,propane, LPG, or naphtha. As in most of the refining and petrochemicaloperations, a steam cracking operation can involve a cryogenicfractionation or a separations section that consists of a series offractionation columns to successively recover various components at highproduct purity.

The present disclosure includes the application of PSA processes to anOCM retrofit of an existing ethylene cracker (e.g., steam cracker).

An example application for OCM combined with a PSA unit involves anexisting petrochemical plant such as a steam cracker is considering lowcost ways to add ethylene capacity. A typical revamp to add capacitycould include addition of, or debottlenecking of, the existingfractionation towers for the entire flow addition for the revamp.However, as shown in FIG. 13, the use of a PSA unit as disclosed hereincan provide a low cost alternative to traditional revamps. An OCM unitwith a PSA unit retrofitted to an existing steam cracker can be aneffective way of adding ethylene capacity at a low marginal cost. Theadvantages of adding a PSA unit include that no additional cryogenicseparation is required for the added capacity. For ethylene revamps, oneof the key areas during debottlenecking may be the refrigeration systemsand/or the fractionation columns, but utilizing the PSA to separate orpre-separate the additional product stream can result in a simpler andeasier debottlenecking. As in shown in FIG. 13, for example, the tailgas from the PSA can be sent to the cracker system where the ethylene isrecovered.

FIG. 13 shows an example of an OCM process integrated with an existingethylene cracker using a PSA system for separations. The OCM reactor1301 takes in methane 1310 and oxygen 1311 and produces an OCM effluent1312 having CO₂, CH₄ and C₂H₄, in some cases amongst other components,such as H₂ and CO. The OCM reaction can be exothermic and can producesteam 1313. The OCM effluent can be compressed in a compressor 1302 andoptionally treated in an acid gas removal system 1303, and fed into apressure swing adsorption (PSA) unit 1304. In some cases the acid gasremoval system may have an additional knock out drum to condense andseparate any condensates and water. It also can include a drier toremove water. The PSA unit can produce a product stream that can includeH₂, CH₄, ethane, CO₂ and CO. The overhead stream 1315 can be fed into amethanation subsystem 1305 (e.g., methanation reactor) to providemethane for the OCM reactor, and some of the overhead stream can bepurged 1316 to a fuel gas system, for example. Additional methane can beprovided by way of a natural gas stream or other methane stream. The PSAtail gas 1317 can comprise most of the ethylene, the content of whichmay range from 50% to 99.9% depending on the process configuration andoperation of the PSA system. The PSA tail gas can also comprise H₂, CO,CO₂, CH₄, ethane, propane, propylene, butanes, butenes, and othercomponents. The process of FIG. 13 can further include an existingethylene cracker 1306. The PSA tail gas can be fractionated usingexisting separations capacity in the ethylene cracker. The heavycomponents can be processed in the fractionation towers of the ethylenecracker, optionally first being compressed in the existing process gascompressor of the ethylene cracker. In some cases, the heavy componentsstream can be routed to the CO₂ removal unit of the existing ethylenecracker subsystem to meet the CO₂ specification. The OCM reactor canreceive a C₂ recycle stream 1319 from the cracker complex.

The combination of a new OCM unit and an existing ethylene cracker canprovide synergistic benefits. It can provide for a low cost alternativeto add ethylene capacity to the existing cracker. In some cases, priorto retrofit of an ethylene cracker with OCM, the entire overhead fromthe existing demethanizer is used as fuel gas, and can now be availableas one of the feeds to the methanation unit. In some cases, thedemethanizer overhead off-gas comprises up to 95% methane, which can beconverted to ethylene in the OCM reactor, hence increasing the totalethylene capacity. In some cases, the hydrogen content in the existingdemethanizer overhead is substantial, and may be enough to meet thehydrogen requirement of the methanation unit.

In some cases, retrofitting an ethylene cracker with OCM reduces (orallows for reduction of) the severity of cracking in the existingcracker, enabling value addition by increasing the production ofpyrolysis gasoline components in the cracker effluent, as the OCMreactor produces the ethylene that may be needed to achieve the totalsystem capacity. The cracker can then be operated on high propylene modeto produce more propylene and at the same time meeting the ethyleneproduction rate by the new OCM unit. This retrofit can result in greaterflexibility for the ethylene producer with respect to the existingcracker operation.

In some instances, the overall carbon efficiency can be increased as themethane and hydrogen from the existing demethanizer off-gases can beutilized to convert the carbon dioxide and carbon monoxide to methane,which is fed to the OCM reactor.

In some instances, ethane and/or propane recycle streams from theexisting cracker can be routed to the OCM unit (e.g., instead of thecracking furnaces). These recycle streams are typically routed to thecracking furnaces where they are cracked to extinction. This can providean advantage over routing the recycle streams to OCM over the crackingfurnace, such as higher selectivity to ethylene in the OCM process.

In certain cases, more than one stages or PSA columns may be employed toachieve higher recovery and higher product purity. As in shown FIG. 14,for example, up to 99.9% recovery is possible using the multi stage PSAunits. An OCM reactor 1402 can receive a methane stream 1410 and anoxygen stream 1411, and react the methane and oxygen to produce higherhydrocarbon products including ethylene in an OCM product stream. TheOCM product stream can be compressed in a first compressor 1403 anddirected to a first PSA separation 1404. The tail gas 1412 from thefirst PSA can be compressed in a second compressor 1405 and fed to asecond PSA separation 1406, the tail gas 1414 from which can becompressed in a third compressor 1407 and separated in a third PSAseparation 1408. The tail gas from the third PSA can be the finalpurified stream 1417 can be directed to a cracker unit, such as anexisting cracker unit, where it can be processed and separated into anethylene product stream 1418, a propylene product stream 1419, and anadditional product stream 1420. PSA product streams 1413, 1415, and 1416can be directed to recycle, such as via a methanation unit 1401, alongwith a demethanizer off gas stream 1421 from the cracker unit. Each PSAstage can be a dual-bed PSA or a multi-bed PSA system.

The application of a PSA unit to OCM systems, standalone or retrofits toexisting facilities exhibits immense potential in terms of cost savingsand ease of integration and retrofit to existing facilities.

ETL Systems

FIG. 15 shows various exemplary configurations for an OCM-ETL process.In the upper left, FIG. 15 shows a stand alone skimmer configuration,where a methane stream 1505 can be directed into an OCM reactor 1501with an oxygen feed 1506 and optionally an ethane feed 1507. The OCMreactor product stream can be directed into a compressor 1502 and theninto an ETL reactor 1503. The ETL product stream can be directed into agas separations unit 1504, where it can be separated into a C₂₊ productstream 1508, a C₅₊ product stream 1509, and an overhead stream 1510comprising methane which can be returned to a pipeline, sold to aconsumer, or otherwise used. In the upper right, FIG. 15 shows a standalone recycle configuration, where a methane feed stream 1518 (e.g.,from a natural gas pipeline) is directed into a treatment unit 1511 andthen into a separations system (e.g., cryogenic) 1512. A methane feedstream 1519 can be directed to an OCM reactor 1513, while anothermethane stream 1520 can be purged or used for power generation. A C₂₊stream 1521 can also be recovered from the separations system. An oxygenfeed stream 1522 and optionally an ethane stream 1523 can also bedirected into the OCM reactor, and the reactor can produce an OCMproduct stream. The OCM product stream can be directed into a compressor1514 and then into an ETL reactor 1515. The ETL product stream can beprocessed in a knockout drum 1516 or other separator to remove a C₅₊product stream 1524. The remaining ETL product stream can be directed toa compressor 1517 and recycled to the treatment unit. In the lower left,FIG. 15 shows a hosted skimmer configuration, where a methane stream1532 can be directed from a separations system 1526 (e.g., cryogenic)into an OCM reactor 1527 with an oxygen feed 1533 and optionally anethane feed 1534. The OCM reactor product stream can be directed into acompressor 1528 and then into an ETL reactor 1529. The ETL productstream can be directed into a gas separations unit 1530, where it can beseparated into a C₂₊ product stream 1535, a C₅₊ product stream 1536, andan overhead stream 1537 comprising methane which can be returned to arecompressor 1531. In the lower right, FIG. 15 shows a hosted recycleconfiguration, where a methane stream is directed into a treatment unit1538 and then into a separations system (e.g., cryogenic) 1539. Amethane feed stream 1546 can be directed to an OCM reactor 1541, whileanother methane stream can be directed to a recompressor 1540. A C₂₊stream 1551 can also be recovered from the separations system. An oxygenfeed stream 1547 and optionally an ethane stream 1548 can also bedirected into the OCM reactor, and the reactor can produce an OCMproduct stream. The OCM product stream can be directed into a compressor1542 and then into an ETL reactor 1543. The ETL product stream can beprocessed in a knockout drum 1544 or other separator to remove a C₅₊product stream 1549. The remaining ETL product stream can be directed toa compressor 1545 and recycled 1550 to the treatment unit.

FIG. 16 shows similar configurations as FIG. 15, with an added pressureswing adsoprtion (PSA) unit to pre-separate the OCM effluent to removemost of the methane, hydrogen, CO and CO₂ from the olefinic stream,which is then fed to the ETL reactor. This can result in a feed to theETL reactor that is concentrated in olefins. Though the process remainssimilar, the entire ETL and separations train becomes considerablysmaller; that is, larger capacities can be achieved in the same set-upor same footprint. In some cases this can improve the ETL reactionoperation. In the upper left, FIG. 16 shows a stand alone skimmerconfiguration, where a methane stream 1606 can be directed into an OCMreactor 1601 with an oxygen feed 1607 and optionally an ethane feed1608. The OCM reactor product stream can be directed into a compressor1602 and then into a PSA unit 1603. A light stream 1609 comprisingmethane, hydrogen, CO and CO₂ can be directed from the PSA back to apipeline, sold to a consumer, or otherwise used. An olefinic stream canbe directed from the PSA to an ETL reactor 1604. The ETL product streamcan be directed into a gas separations unit 1605, where it can beseparated into a C₂₊ product stream 1610, a C₅₊ product stream 1611, andan overhead stream 1612 comprising methane which can be returned to apipeline, sold to a consumer, or otherwise used. In the upper right,FIG. 16 shows a stand alone recycle configuration, where a methane feedstream 1628 (e.g., from a natural gas pipeline) is directed into atreatment unit 1620 and then into a separations system (e.g., cryogenic)1621. A methane feed stream 1629 can be directed to an OCM reactor 1622,while another methane stream 1630 can be purged or used for powergeneration. A C₂₊ stream 1631 can also be recovered from the separationssystem. An oxygen feed stream 1632 and optionally an ethane stream 1633can also be directed into the OCM reactor, and the reactor can producean OCM product stream. The OCM product stream can be directed into acompressor 1623, and at least a portion 1634 of the OCM product streamcan be directed from the compressor into a PSA unit 1624. A light stream1635 comprising methane, hydrogen, CO and CO₂ can be directed from thePSA back to the treatment unit. An olefinic stream 1636 can be directedfrom the PSA to an ETL reactor 1625. The ETL product stream can beprocessed in a knockout drum 1626 or other separator to remove a C₅₊product stream 1637. The remaining ETL product stream can be directed toa compressor 1627 and recycled to the treatment unit. In the lower left,FIG. 16 shows a hosted skimmer configuration, where a methane stream1647 can be directed from a separations system 1640 (e.g., cryogenic)into an OCM reactor 1641 with an oxygen feed 1648 and optionally anethane feed 1649. The OCM reactor product stream can be directed into acompressor 1642 and then into and then into a PSA unit 1643. A lightstream 1650 comprising methane, hydrogen, CO and CO₂ can be directedfrom the PSA to a recompressor 1646. An olefinic stream can be directedfrom the PSA to an ETL reactor 1644. The ETL product stream can bedirected into a gas separations unit 1645, where it can be separatedinto a C₂₊ product stream 1651, a C₅₊ product stream 1652, and anoverhead stream 1653 comprising methane which can be returned to therecompressor. In the lower right, FIG. 16 shows a hosted recycleconfiguration, where a methane stream is directed into a treatment unit1660 and then into a separations system (e.g., cryogenic) 1661. Amethane feed stream 1669 can be directed to an OCM reactor 1663, whileanother methane stream can be directed to a recompressor 1662. A C₂₊stream 1677 can also be recovered from the separations system. An oxygenfeed stream 1670 and optionally an ethane stream 1671 can also bedirected into the OCM reactor, and the reactor can produce an OCMproduct stream. The OCM product stream can be directed into a compressor1664 and at least a portion 1672 of the OCM product stream can bedirected from the compressor into a PSA unit 1665. A light stream 1673comprising methane, hydrogen, CO and CO₂ can be directed from the PSAback to the treatment unit. An olefinic stream 1674 can be directed fromthe PSA to an ETL reactor 1666. The ETL product stream can be processedin a knockout drum 1667 or other separator to remove a C₅₊ productstream 1675. The remaining ETL product stream can be directed to acompressor 1668 and recycled 1676 to the treatment unit.

The ETL reactor can be a tubular, packed bed, moving bed, fluidized bed,or other reactor type. An ETL reactor can be an isothermal or adiabaticreactor. The ETL system can benefit from a feed concentrated in olefins.The ETL reactor system can use a recycle stream to control and moderatethe temperature increase in the reactor bed due to the highly exothermicnature of the ETL reactions. ETL systems are described in, for example,U.S. patent application Ser. No. 14/591,850, which is entirelyincorporated herein by reference.

In certain embodiments, one or more of the fractionation towers can bedeemed redundant if using the PSA, as an example, a demethanizer may notbe required and the sales gas or purge gas to fuel can be sent from thePSA itself.

Retrofit Applications for Midstream and Refining

Systems, such as those of FIG. 17, can be integrated with an existinggas processing plant where one or more of the existing subsystems can beutilized. The utilization may arise from the fact that the existingsubsystems are no longer used, or have an additional capacity availableto allow for the integration.

FIG. 17 shows an exemplary application of an OCM-ETL system using a PSAsystem for pre-separations to an existing gas processing plant, whereone or more existing sub systems may be utilized. As shown in FIG. 17,the existing separations sub-system can be integrated with the OCM-ETLsystem to add value by converting natural gas to higher value liquidhydrocarbons. The PSA unit can be used to pre-separate the lightercomponents like methane, hydrogen, carbon monoxide, carbon dioxide,ethane, and other components, and the olefin rich stream can be sent tothe ETL reactor that converts the olefins to higher molecular weightliquid hydrocarbons. One advantage of using a PSA system is thereduction in net additional feed to the existing separation system,which can be de-bottlenecked easily. If the separation system is nolonger in use, addition of a PSA can bring about larger total capacitiesthat can be achieved by adding larger OCM-ETL systems. A natural gasstream 1720 can be directed to a treatment unit 1701 and then into aseparations system (e.g., cryogenic) 1702. At least portion of a methanestream 1724 from the separations unit can be directed to an OCM reactor1705, while a portion of the methane stream can be directed to acompressor 1703 and used as sales gas 1721 or other purposes. A higherhydrocarbon stream can be directed from the separations system to a C₂removal unit 1704, which can produce a natural gas liquids stream 1722and a C₂ stream 1723. The C₂ stream can be fed into the OCM reactor withthe methane stream and an oxygen stream 1725, and reacted to form higherhydrocarbon products including ethylene. The OCM product stream can bedirected into a heat recovery system 1706, which can generate a highpressure superheated (HPSH) steam stream 1726. The OCM product streamcan then be directed to a knockout drum to recover a condensate stream1727. The OCM product stream can then be directed to a compressor 1708,which can operate using the HPSH steam stream. From the compressor, theOCM product stream can be directed to a PSA unit 1709. From the PSAunit, light stream comprising methane, hydrogen, CO and CO₂ can bedirected to a methanation unit 1710, and an olefinic stream can bedirected to an ETL reactor 1711 and reacted to form higher hydrocarbonproducts. The ETL product stream can be directed to a heat recovery unit1712, where boiler feed water (BFW) 1728 can be heated, at least aportion of which can be fed 1729 to the heat recovery unit 1706. The ETLproduct stream can then be directed to another knockout drum 1713. Theoverhead stream from the knockout drum can be directed to a lowtemperature separations unit 1714, while the bottoms stream from theknockout drum can be directed to a C₄ removal unit 1715, which canproduce a C₄ stream 1730 and a C₅₊ stream 1731. Overhead from the lowtemperature separations unit, as well as product from the methanationreactor, can be directed back to the compressor 1703.

OCM-ETL systems of the present disclosure can be integrated into andcombined into conventional NGL extraction and NGL fractionation sectionsof a midstream gas plant. Where NGLs in the gas stream are declining (orgas is dry), the deployment of OCM-ETL can utilize an existing facilityto produce additional liquid streams. The implementation of OCM-ETL canallow for the generation of on specification “pipeline gas.” Theproducts from the facility can be suitable for use (or on specificationor “spec”) as pipeline gas, gasoline product, hydrocarbon (HC) streamswith high aromatic content, and mixed C₄ products. The PSA systemsdiscussed above can be employed to separate, pre-separate or purify thehydrocarbon feed streams in the integrated NGL OCM-ETL system. FIG. 18shows an exemplary NGL extraction facility integrated with an OCM-ETLsystem. As shown in FIG. 18, for example, the feed to the PSA 1802 canbe the net incoming gas from the treatment system 1801, which can treata methane stream (e.g., natural gas) 1810. The PSA system can separatethe feed to the OCM reactor 1803, which is mostly methane and lightercomponents with some ethane to utilize a PBC section of the OCM reactor,and the feed to the ETL reactor 1805, which can first be processed in anatural gas liquids extraction system 1804. The feed to the ETL systemcan be the PSA tail gas and OCM effluent comprising ethylene, propylene,ethane, propane, hydrogen, methane, and other components. In some cases,the OCM effluent can be directly fed to the ETL reactor. In some casesthe OCM effluent is hydrogenated and fed to the ETL system. In somecases, as shown for example in FIG. 18, the OCM effluent is fed back tothe PSA unit for separation; additional natural gas 1811 can be added,and a stream can be recovered 1812 (e.g., for use as pipeline gas). Insome examples, the system may have a methanation unit that takes in theeffluent from ETL reactor or OCM reactor and converts the CO, CO₂ and H₂to methane, thereby further increasing the carbon efficiency of theprocess. The existing NGL extraction and product fractionation 1806sub-systems can then be used to fractionate the final products,including into a mixed C₄ stream 1814 and a C₅₊ product stream 1815.

Refining

Refinery gas typically contains valuable components like hydrogen,methane, ethane, ethylene, propane, propylene, and butane. Mostcommonly, refinery off-gases (ROG) are exported to the fuel gas system,thereby losing the value of the components contained therein. TheOCM-ETL process can be used to improve the value of products as the OCMconverts the methane to ethylene and the ETL converts olefins (e.g.,those existing in the ROG and those generated by OCM) to higher valueliquids as C₄ components, gasoline blends, or aromatic components.

FIG. 19 shows an exemplary PSA unit integrated to a refinery processscheme. A refinery gas plant 1901 can receive gas 1910 from cracking orother units. The PSA unit 1903 (after, for example, treatment of the gasin a treatment unit 1902) can separate components in refinery gas plantoff gas to methane and a C₂₊ cut which contains most or all of theolefinic materials. The methane can be used as refinery fuel 1911 and/ordirected to an OCM unit 1904 with post-bed cracking The OCM feed can besupplemented with additional natural gas 1912. The olefinic materialscan be directed to an ETL reactor 1905. The OCM effluent can also berouted to the PSA where the olefins produced in the OCM are also sent tothe ETL reactor. In some cases, the OCM effluent can be routed to theETL reactor. In some cases, the OCM effluent may be hydrogenated beforebeing sent to the PSA unit or ETL reactor. Some techniques may dictatethe use of a cryogenic demethanizer in place of the PSA, but theapplication of PSA to pre-separate the refinery off-gas into a productstream and a tail gas stream containing the heavier hydrocarbons whichis the feed to ETL reactor can result in significant cost savings. Theproduct stream can contain methane, ethane, CO, CO₂, and othercomponents, with of each component from 1 to 99%. A C₃₊ stream 1913 fromthe refinery gas plant can be directed to a product fractionation system1906, which can provide a C₂/C₃ stream 1914 (which can be directed tothe OCM reactor), an iC₄ stream 1915, a gasoline blend stream 1916,and/or a kerosene/jet stream 1917.

As shown in FIG. 20, in some cases the system can have a methanationunit to further improve the carbon efficiency of the process. A refinerygas plant 2001 can receive gas 2010 from cracking or other units. ThePSA unit 2003 (after, for example, treatment of the gas in a treatmentunit 2002) can separate components in refinery gas plant off gas tomethane and a C₂₊ cut which contains most or all of the olefinicmaterials. The methane can be used as refinery fuel 2011 and/or directedto a methanation unit 2004, and then to an OCM reactor 2005 withpost-bed cracking The methanation feed can be supplemented withadditional natural gas 2012. The olefinic materials can be directed toan ETL reactor 2006. The OCM effluent can be routed to the ETL reactor.In some cases, the OCM effluent can also be routed to the PSA where theolefins produced in the OCM are also sent to the ETL reactor. In somecases, the OCM effluent may be hydrogenated before being sent to the PSAunit or ETL reactor. Some techniques may dictate the use of a cryogenicdemethanizer in place of the PSA, but the application of PSA topre-separate the refinery off-gas into a product stream and a tail gasstream containing the heavier hydrocarbons which is the feed to ETLreactor can result in significant cost savings. The product stream cancontain methane, ethane, CO, CO₂, and other components, with of eachcomponent from 1 to 99%. A C₃₊ stream 2013 from the refinery gas plantcan be directed to a product fractionation system 2007, which canprovide a C₂/C₃ stream 2014 (which can be directed to the OCM reactor),an iC₄ stream 2015, a gasoline blend stream 2016, and/or a kerosene/jetstream 2017.

Methods and systems of the present disclosure can be combined with ormodified by other methods and systems, such as those described in U.S.patent application Ser. No. 14/591,850, filed Jan. 7, 2015, nowpublished as U.S. Patent Pub. No 2015/0232395; U.S. patent applicationSer. No. 13/936,783, filed Jul. 8, 2013, now published as U.S. PatentPub. No. 2014/0012053; U.S. patent application Ser. No. 13/936,870,filed Jul. 8, 2013, now published as U.S. Patent Pub. No. 2014/0018589;U.S. patent application Ser. No. 13/900,898, filed May 23, 2013, nowpublished as U.S. Patent Pub. No 2014/0107385; U.S. patent applicationSer. No. 14/553,795, filed Nov. 25, 2014, now published as U.S. PatentPub. No. 2015/0152025; U.S. patent application Ser. No. 14/592,668,filed Jan. 8, 2015, now published as U.S. Patent Pub. No. 2015/0210610;and U.S. patent application Ser. No. 14/789,953, filed Jul. 1, 2015,each of which is entirely incorporated herein by reference.

While preferred embodiments of the present invention have been shown anddescribed herein, it will be obvious to those skilled in the art thatsuch embodiments are provided by way of example only. It is not intendedthat the invention be limited by the specific examples provided withinthe specification. While the invention has been described with referenceto the aforementioned specification, the descriptions and illustrationsof the embodiments herein are not meant to be construed in a limitingsense. Numerous variations, changes, and substitutions will now occur tothose skilled in the art without departing from the invention.Furthermore, it shall be understood that all aspects of the inventionare not limited to the specific depictions, configurations or relativeproportions set forth herein which depend upon a variety of conditionsand variables. It should be understood that various alternatives to theembodiments of the invention described herein may be employed inpracticing the invention. It is therefore contemplated that theinvention shall also cover any such alternatives, modifications,variations or equivalents. It is intended that the following claimsdefine the scope of the invention and that methods and structures withinthe scope of these claims and their equivalents be covered thereby.

1. A method for generating higher hydrocarbon(s) from a streamcomprising compounds with two or more carbon atoms (C₂₊), comprising:(a) introducing methane and an oxidant into an oxidative coupling ofmethane (OCM) reactor that has been retrofitted into a system comprisingan ethylene-to-liquids (ETL) reactor, wherein said OCM reactor reactssaid methane with said oxidant to generate a first product streamcomprising said C₂₊ compounds; (b) directing said first product streamto a pressure swing adsorption (PSA) unit that recovers at least aportion of said C₂₊ compounds from said first product stream to yield asecond product stream comprising said at least said portion of said C₂₊compounds; (c) directing said second product stream to said ETL reactor;and (d) generating said higher hydrocarbon(s) from said at least saidportion of said C₂₊ compounds in said ETL reactor.
 2. The method ofclaim 1, further comprising: recovering a light stream comprising (i)hydrogen and (ii) carbon monoxide (CO) and/or carbon dioxide (CO₂) fromsaid PSA unit and recycling said light stream to said OCM reactor;directing at least a portion of said light stream into a methanationunit that reacts said hydrogen and said CO and/or CO₂ to produce amethanation product stream comprising methane; and directing saidmethanation product stream into said OCM reactor.
 3. The method of claim1, further comprising recovering C₂ and/or C₃ compounds from said secondproduct stream and directing said C₂ and/or C₃ compounds to said OCMreactor.
 4. The method of claim 1, wherein said OCM reactor furthercomprises a post-bed cracking (PBC) unit.
 5. A method for generatingcompounds with two or more carbon atoms (C₂₊ compounds), comprising: (a)directing oxygen (O₂) and methane (CH₄) into an oxidative coupling ofmethane (OCM) reactor that reacts said O₂ and CH₄ in an OCM process toyield a product stream comprising (i) C₂₊ compounds including ethylene(C₂H₄) and (ii) carbon monoxide (CO) and/or carbon dioxide (CO₂); and(b) directing said product stream from said OCM reactor into aseparations system that employs a refrigeration unit having arefrigerant that includes methane from said product stream, to enrichsaid C₂₊ compounds in said product stream.
 6. The method of claim 5,wherein said product stream is directed into said separations systemthrough one or more additional units.
 7. The method of claim 5, furthercomprising separating methane from said product stream for use in saidrefrigeration unit.
 8. The method of claim 5, further comprisingdirecting CO and/or CO₂ from said product stream to a methanationreactor that reacts said CO and/or CO₂ to yield a methanation productstream comprising methane.
 9. The method of claim 8, further comprisingdirecting at least a portion of said methane in said methanation productstream to said OCM reactor.
 10. The method of claim 5, furthercomprising separating said product stream into (i) an ethylene productstream comprising ethylene and (ii) a C₃₊ product stream comprisingcompounds with three or more carbon atoms (C₃₊ compounds).
 11. Themethod of claim 5, further comprising directing ethane from said productstream to said OCM reactor.
 12. The method of claim 5, furthercomprising, prior to directing said product stream into said separationssystem, compressing said product stream.
 13. A method for generatingcompounds with two or more carbon atoms (C₂₊ compounds), comprising: (a)directing oxygen (O₂) and methane (CH₄) into an oxidative coupling ofmethane (OCM) reactor that reacts said O₂ and CH₄ in an OCM process toyield a product stream comprising (i) C₂₊ compounds including ethylene(C₂H₄) and (ii) carbon monoxide (CO) and/or carbon dioxide (CO₂); and(b) directing said product stream from said OCM reactor into aseparations system that employs a complexation unit having acomplexation catalyst that forms pi complexes with said ethylene in saidproduct stream, to enrich said C₂₊ compounds in said product stream. 14.The method of claim 13, wherein said product stream is directed intosaid separations system through one or more additional units.
 15. Themethod of claim 13, further comprising using said complexation unit toremove one or more impurities from said product stream, wherein saidimpurities are selected from the group consisting of CO₂, sulfurcompounds, acetylenes, and hydrogen.
 16. The method of claim 13, whereinsaid complexation catalyst includes one or more metals selected from thegroup consisting of silver and copper.
 17. The method of claim 13,further comprising directing CO and/or CO₂ from said product stream to amethanation reactor that reacts said CO and/or CO₂ to yield amethanation product stream comprising methane.
 18. The method of claim17, further comprising directing said methane in said methanationproduct stream to said OCM reactor.
 19. The method of claim 13, furthercomprising separating said product stream into (i) an ethylene productstream comprising ethylene and (ii) a C₃₊ product stream comprisingcompounds with three or more carbon atoms (C₃₊ compounds).
 20. Themethod of claim 13, further comprising directing ethane from saidproduct stream to said OCM reactor.
 21. The method of claim 13, furthercomprising, prior to directing said product stream into said separationssystem, compressing said product stream.
 22. A method for generatingcompounds with two or more carbon atoms (C₂₊ compounds), comprising: (a)directing oxygen (O₂) and methane (CH₄) into an oxidative coupling ofmethane (OCM) reactor that reacts said O₂ and CH₄ in an OCM process toyield a product stream comprising (i) C₂₊ compounds including ethylene(C₂H₄) and (ii) carbon dioxide (CO₂); and (b) directing said productstream from said OCM reactor into a separations system that employs aCO₂ separation unit to separate said CO₂ from said product stream, toenrich said C₂₊ compounds in said product stream, which CO₂ separationunit employs (i) sorbent or solvent separation of CO₂, (ii) membraneseparation of CO₂, or (iii) cryogenic or low temperature separation ofCO₂ having an operating temperature greater than a boiling point ofmethane and less than a boiling point of CO₂.
 23. The method of claim22, wherein said product stream is directed into said separations systemthrough one or more additional units.
 24. The method of claim 22,wherein said sorbent or solvent separation of CO₂ employs an amine basedabsorption system, a Benfield process, diethanolamine, glycoldimethylether, propylene carbonate, Sulfinol, a zeolite, or activecarbon. 25.-31. (canceled)
 32. The method of claim 22, wherein said CO₂separation system comprises a membrane CO₂ separation system.
 33. Themethod of claim 22, wherein said membrane separation of CO₂ employs apolymeric membrane, metallic membrane, ceramic membrane, poly ionicliquid membrane, supported ionic liquid membrane, polyetherimidemembrane, or hybrid membrane comprising a membrane supporting a solventor sorbent. 34.-39. (canceled)
 40. The method of claim 22, furthercomprising directing said CO₂ from said product stream to a methanationreactor that reacts said CO₂ to yield a methanation product streamcomprising methane.
 41. The method of claim 40, further comprisingdirecting said methane in said methanation product stream to said OCMreactor.
 42. The method of claim 22, further comprising separating saidproduct stream into (i) an ethylene product stream comprising ethyleneand (ii) a C₃₊ product stream comprising compounds with three or morecarbon atoms (C₃₊ compounds).
 43. The method of claim 22, furthercomprising directing ethane from said product stream to said OCMreactor.
 44. The method of claim 22, further comprising, prior todirecting said product stream into said separations unit, compressingsaid product stream.
 45. A method for generating compounds with two ormore carbon atoms (C₂₊ compounds), comprising: (a) directing water intoan electrolysis unit that electrolyzes said water to yield oxygen (O₂)and hydrogen (H₂); (b) directing said O₂ from said electrolysis unit andmethane (CH₄) into an oxidative coupling of methane (OCM) reactor thatreacts said O₂ and CH₄ in an OCM process to yield a product streamcomprising (i) C₂₊ compounds, including ethylene (C₂H₄) and (ii) carbonmonoxide (CO) and/or carbon dioxide (CO₂); (c) directing at least aportion of said CO and/or CO₂ from said product stream and said H₂ fromsaid electrolysis unit into a methanation reactor that reacts said H₂and said CO and/or CO₂ to yield CH₄; and (d) directing at least aportion of said CH₄ from said methanation reactor to said OCM reactor.46.-48. (canceled)
 49. A method for generating compounds with two ormore carbon atoms (C₂₊ compounds), comprising: (a) directing oxygen (O₂)and methane (CH₄) into an oxidative coupling of methane (OCM) reactorthat reacts said O₂ and CH₄ in an OCM process to yield a product streamcomprising (i) C₂₊ compounds including ethylene (C₂H₄) and (ii) carbondioxide (CO₂); (b) directing said product stream from said OCM reactorinto a separations system that employs a CO₂ separation unit thatseparates said CO₂ from said product stream to enrich said C₂₊ compoundsin said product stream; and (c) directing at least a portion of said CO₂separated in (b) to said OCM reactor.
 50. (canceled)
 51. A method forgenerating compounds with two or more carbon atoms (C₂₊ compounds),comprising: (a) directing oxygen (O₂) and methane (CH₄) into anoxidative coupling of methane (OCM) reactor that reacts said O₂ and CH₄in an OCM process to yield a product stream comprising C₂₊ compoundsincluding ethylene (C₂H₄) and heat; (b) using an evaporator to transferat least a portion of said heat from said product stream to an organicworking fluid in a closed fluid flow cycle as part of an organic Rankinecycle (ORC) system, to evaporate said organic working fluid, whichclosed fluid flow cycle includes said evaporator, a turbine, acondenser, and a pump; (c) directing said organic working fluidevaporated in (b) to said turbine to generate power; (d) directing saidorganic working fluid from said turbine to said condenser that condensessaid organic working fluid; and (e) directing said organic working fluidcondensed in (d) to said pump.
 52. (canceled)
 53. (canceled)
 54. Amethod for generating compounds with two or more carbon atoms (C₂₊compounds), comprising: (a) directing oxygen (O₂) and methane (CH₄) intoan oxidative coupling of methane (OCM) reactor that reacts said O₂ andCH₄ in an OCM process to yield a product stream comprising (i) C₂₊compounds including ethylene (C₂H₄) and heat; (b) transferring at leasta portion of said heat from said product stream to a thermoelectricpower generator; and (c) with the aid of said heat, using saidthermoelectric power generator to generate power.
 55. (canceled) 56.(canceled)